[161]

NEURAL PATHWAYS IN LACTATION

By J. T. EAYRS AND R. M. BADDELEYDepartment of Anatomy, University of Birmingham

It has long been recognized that suckling plays an important part in maintaininglactation. The principal underlying factor was at one time thought to be the with-drawal of milk, for a progressive involution of mammary tissues occurs when intra-alveolar pressures remain high (Petersen & Rigor, 1932; Selye & McKeown, 1934).More recent evidence, however (for reviews see Folley, 1947; Cross, 1955), suggeststhat suckling influences the course of lactation, not so much through a local effect(though such may be present) as by way of a system of neuro-endocrine reflexesinvolving the hypothalamus and pituitary gland.The essential participation of the nervous system in this regulatory mechanism

was first shown by Ingelbrecht (1935) who cut the spinal cord in the rat and foundthat, while lactation ceased when suckling was restricted to nipples innervated fromsegments caudal to the site of lesion, it continued when nipples cranial to this levelwere available. Little further has been done, however, to elucidate the anatomyand mode of function of the nervous pathways involved. Such evidence as is avail-able suggests that the sensory receptors, though unidentified, are located in thenipples (Hooker & Williams, 1940) and that, since total sympathectomy does notinhibit lactation (Bacq, 1932), stimuli arising in these receptors are unlikely to beconveyed through autonomic channels. Central pathways to the hypothalamus, andthose by which the hypothalamus regulates pituitary activity, remain obscure.The present paper describes experiments, a preliminary account of which has

appeared elsewhere (Eayrs & Baddeley, 1955), undertaken to study the course of thispathway through the neuraxis.

MATERIALS AND METHODS

AnimalsOne hundred and four virgin rats of the inbred 'Birmingham' strain were used. Eachrat was mated, and on the day following the birth of its litter (except where other-wise stated) was partially thelectomized, i.e. all nipples except two pairs were excised,the young being reduced in number to two males and two females. The gain in weightof these four rats during the first 16 days of life (23 days in the case of group 1) wasused as a measure of the lactational performance of the mother. In experimentswhere the gains in weight of successive litters were compared, the doe was alwaysremated with the same male.

Operational proceduresThe rats were divided into eight groups. Groups 1 and 2 were treated as described

above and underwent no further operation. In the remaining groups the lactating11 Anat. 90

162 J. T. Eayrs and R. M. Baddeleyability of the doe was measured after the following lesions, summarized in Table 1,had been made to the nervous system under avertin anaesthesia.

(a) Group 3. Exposure of spinal cordThe meninges were exposed either by removing the ligamentum flavum between

the laminae of the 6th and 7th cervical vertebrae, or by removing a pair of vertebrallaminae in the thoraco-lumbar region. The dura mater was then punctured with asharp-pointed knife and the incision widened with forceps.

Table 1. Summary of operational proceduresTreatments

No. of A-Group rats To nipples To nervous system

1 8 None2 12 None3 6 Spinal cord exposed4 7 Section of dorsal roots5 12 Section of dorsal funiculi6a 5 . Section of lateral funiculi

Partial thelectomy; four pups reared atrteetmSb 4 ~~~ontwo pairs of nipples after thelectomy6b ontopis fnplsSection of lateral funiculi

before thelectomy6c 10 As for 6a but smaller

lesions7 6 Combined section of dorsal

and ventral funiculi8a 7 ( Ipsilateral hemisection of

Partial thelectomy; three pups the cord after thelectomy8b 6 reared on three nipples-two trafter helectomyabdominal and one inguinal on . c a

8c 21 same side Ipsi- and contralateralhemisection of the cordbefore thelectomy

(b) Group 4. Section of dorsal rootsThe deep muscles of the back were retracted and the vertebral laminae removed

from L2 to T7. The spinal theca was then opened, and six consecutive pairs of sub-jacent dorsal roots cut close to the spinal cord. The bleeding which usually occurredboth from the bone and from the superficial veins of the spinal cord was controlledby applying pledgets of cotton-wool soaked in thrombin.

(c) Groups 5-8. Section of tracts in spinal cordThe spinal cord was exposed in the manner described for group 3, and bilateral

lesions made with a discission knife (see Text-fig. 1) in the dorsal (group 5) and lateral(group 6) funiculi and in the dorsal and ventral funiculi combined (group 7). In therats of group 8, the spinal cord was hemisected, the nipples of some of these animalsbeing removed ipsilaterally, and in others contralaterally. Further details of theseprocedures, including the levels at which the several lesions were made, are, forconvenience, given with the results.

Preparation of tissuesAt the end of the experiments, the rats of groups 3-8 were killed in chloroform

vapour. The central nervous system was removed and prepared for histologicalexamination by Marchi's original method or by the Swank & Davenport (1935)

Neural pathways in lactation 163modification, representative celloidin sections being cut at 20,s through the spinalcord, medulla, pons and mid-brain. In addition, in the rats of group 8, that part ofthe vertebral column containing the lesion to the spinal cord was decalcified andembedded in paraffin wax. Longitudinal sections 15#t thick were cut and stainedwith cresyl violet or impregnated with silver (method of Romanes, 1950).

Text-fig. 1. Schematic diagrams showing manner in which lesions were made to spinal cordof rats in groups 5-8.

RESULTS

(a) Control operations-standard of lactational performance(i) Thelectomy onlyThe results (Table 2a, b) serve to establish the pattern of growth of successive

litters reared by the same female. They show that, in the absence of injury to thenervous system, such litters did not grow at the same rate, the mean weight of thesecond litter being significantly greater (11 % in group 1 and 13% in group 2) thanthat of the first. The growth of the third litter did not, however, differ from that ofthe second. The rate of growth of these litters proved independent of which pairs ofnipples were available for suckling.

11-2

164 J. T. Eayrs and R. M. Baddeley

(ii) Mock operationMock operation in the region of the spinal cord caused some impairment of

lactation (Table 2c). The spinal theca in the rats of group 3 was opened on the dayfollowing the birth of the second litter, and although this litter grew slightly betterthan the first, the marked improvement in lactation characteristic of the unoperatedrat (groups 1 and 2) was considerably reduced. The third litter grew slightly, butnot significantly, better than the second.

(b) Effect of bilateral lesions to the nervous system(i) Section of dorsal rootsThe dorsal roots were cut in seven rats (group 4) before mating and before any

nipples had been removed. Two of these rats proved sterile; the other five were allowed

Table 2. Lactational ability of rats-effect of partial thelectomy both aloneand combined with lesions to the nervous system

Mean increase in weightof litter of four rats from

No.of

Serial Group rats Treatment(a) 1 8 Partial thelectomy

(b) 2 12 Partial thelectomy

(c) 3 6 Partial thelectomyPartial thelectomy combinedwith thecal incision

(d) 5 12 Partial thelectomyPartial thelectomy combinedwith section of dorsalfuniculi

(e) 6c 8 Partial thelectomyPartial thelectomy combinedwith section of lateralfuniculi

I

birth to 17 days old Differences + standard errom(g. rat) A

A& 5 Litter 1 Litter 2Litter 1 Litter 2 Litter 3 litter 2 litter 335-2 39-2 - 4-0+1-66

t= 2-425P=0-05-002

23-2 26-3 26-3 3-1 + 0-46 Nilt=6-682P<0-001

21-1 - 1-3+1-21 1-0+1-2122-4 23-4 t=1 070 t=0-829

P=0*4-0-3 P=0-5 0-22-6 - 2-6+0-89 1-4+0-89- 20-0 21-4 t=2-880 t=1-523

P= 0-02-001 P=o02-0:

21X1 2-4+0-8018-8 t=2-999

P= 0-02-001

to rear their full litters for a few days until it was established, by examining thestomachs of the young, that ample quantities ofmilk were being obtained. The extentof the anaesthetic region of the abdominal wall was then determined by pinching thenipples and surrounding skin with forceps under very light ether or avertin anaes-thesia, and noting the presence or absence of a scratch reflex. All nipples outside thiszone were then removed and the litter reduced to a size of one pup to each remainingnipple.

Lactation ceased altogether once the nipples outside the anaesthetic area had beenremoved, in spite of the fact that the remaining nipples were suckled vigorously andthat the doe continued to nurse the young. After remating, two of these rats didsucceed in rearing subsequent litters-one its third, and the other its fourth andfifth. This recovery coincided with a restoration of cutaneous sensitivity in theregion of the inguinal nipples.

Q

Neural pathways in lactation

(ii) Section of dorsal funiculiLactation did not cease after section of the dorsal funiculi, but was impaired more

severely than after mock operation alone (Table 2 d). The operation was carried outat the level of C7 on the twelve rats of group 5 on the day following the birth of thesecond litter, which grew significantly less well (11 %) than did the first. The growthof the third litter showed a partial recovery to a level half-way between that of thefirst and the second.

(iii) Section of lateralfuniculiSmall lesions made to the lateral funiculi were associated with an impairment to

lactation comparable with that following section of the dorsal funiculi (Table 2e).Lactation ceased altogether, however, as a result of more extensive lesions (seeText-fig. 1) which at the same time interfered with micturition.These observations were made on the nineteen rats of group 6 (see Table 1). Five

of these (sub-group 6a) were rats whose performance had already been studied ingroup 1. Each had accordingly only four nipples at the time of operation. The lateralfuniculi were cut at the level of T6 on the day following the birth of their thirdlitter, and the litter was at the same time reduced to four in number. Three of theserats failed to rear their young altogether; the remaining two reared their litters, but

Table 3Operational procedure Physical disabilities Lactational performance

Sub-group 6b Bilateral lesions, similar Paresis. Bladder Raised litters before(four rats) to those of non-lactating dysfunction partial thelectomy;

rats of sub-group 6a, lactation ceased aftermade before thelectomy removal of all but the

abdominal and ingui-nal pairs of nipples

Sub-group 6c Smaller bilateral lesions Paresis. No bladder Eight rats raised litters(ten rats) similar to those of dysfunction but less successfully

lactating rats of sub- than pre-operationallygroup 6a, made after (Table 2e). Two ratsthelectomy failed to lactate

the growth of their young was considerably less than that of their previous litters.All suffered from physical disabilities, e.g. paresis of the hind-limbs, disturbedgait and retention of urine, but the condition of the two which lactated was betterthan that of those which did not.As a result of these findings, two further experiments carried out to determine

whether the failure of lactation was directly due to the interruption of spinal path-ways or was caused indirectly by the inability of the doe to nurse the young. Thesegave the results shown in Table 3.

It may be inferred from the combined results of these experiments: (i) that thearrest of lactation in animals with extensive lesions in the lateral funiculi of thespinal cord is directly due to interference with the afferent nervous pathways con-cerned with maintaining lactation, rather than to indirect causes; and (ii) that thesepathways are situated deep in the spinal cord and are anatomically closely associatedwith those concerned in the control of the bladder.

165

166 J. T. Eayrs and R. M. Baddeley

(iv) Combined section of dorsal and ventral funiculiThis procedure varied in its effect, resulting either in an impairment of lactation

considerably more severe than that which followed section of the dorsal funiculialone, or in complete arrest of lactation. Lesions were made to the spinal cords of thesix rats used (group 7) at the level of C 7. Three of these rats failed to raise theirsecond (post-operative) litter. The other three raised their second litter, but in allcases the young gained less weight (34 %) than those of the first (pre-operative)litter. All rats but one, however, reared their third litters but the growth of theselitters was markedly depressed (40 %) relative to that of the first.

(c) Effect of unilateral lesions-total hemisection of the spinal cordThe effect of complete hemisection depended upon whether nipples were available

on the same side as that of the lesion (ipsilateral hemisection) or on the opposite side(contralateral hemisection-see Text-fig. 1). With a few exceptions rats which hadundergone ipsilateral hemisection failed to lactate while those with contralaterallesions succeeded in raising their litters.

Thirty-four rats in group 8 were used for this experiment. Of these, 13 (sub-groups8 a and 8 b) had successfully reared a reduced litter of three young on three nipples(two abdominal and one inguinal on the same side) before operation. In seven rats(sub-group 8 a), the spinal cord was cut ipsilaterally on the day following the birth ofthe second litter which was at the same time reduced to three in number. Six ofthese rats failed to rear their litters; one succeeded, but its young at 17 days oldweighed considerably less than did those of its first litter. Contralateral lesions weremade to the spinal cords of the six rats of sub-group 8 b. Five of these rats raised theirlitters (though less successfully than their first litters), and one failed.

In the remaining twenty-one rats (sub-group 8c), which had previously reared afull-sized litter satisfactorily, the experimental procedure was varied, the spinalcords being hemisected (10 at cervical, 11 at thoracic level) and the animals returnedto their litters without interference with the nipples. When lactation was firmlyre-established, and when the rat had recovered from any obvious motor disabilityand was nursing the young in a well-made nest, partial thelectomy was performedleaving only the abdominal and inguinal nipples on one side. The size of the litterwas at the same time reduced to three. Irrespective of whether the lesion was madein the cervical or thoracic region of the cord, all the rats with contralateral lesions(7) continued to lactate satisfactorily. On the other hand, the young of ten out ofthe fourteen rats which had received ipsilateral lesions died of inanition, no milkbeing seen in their stomachs later than 12 hr. after thelectomy.These findings are thus substantially similar to those for sub-groups 8a and 8 b,

with which they may therefore be combined as under:

Failed toLesion Lactated lactate Total Statistics

Ipsilateral hemisection 5 16 21 X2= 12X45Contralateral hemisection 12 1 13 P <0.01

It may be inferred from these figures that the stimuli concerned in maintaining lac-tation are for the most part conveyed ipsilaterally through the spinal cord. At the

Neural pathways in lactation 167same time it was not possible, as a result of histological examination of the site oflesion (see Text-fig. 2), to explain the lactational ability of the five rats which rearedtheir young after ipsilateral hemisection of the spinal cord in terms of inadequatedestruction of nervous tissue. It seems likely, therefore, that the pathway is bi-laterally represented, containing a contralateral component which is adequate tomaintain lactation after interruption of the main pathway, but whose destructionhas only a depressing effect.

(d) Histological examinationA study of the degeneration which had occurred in the spinal cord and brain

stem, other than confirming the extent of the various lesions, provided little newinformation. The tissues of the mock-operated rats (group 3) showed no sign of

Rat no. 105 153 155 I 57

Rat no. 107 118 123 156

Text-fig. 2. Comparison of extent of ipsilateral lesions to the spinal cord (hatched) in: above-fourrats which failed to lactate; below-four rats (typical sample) which lactated satisfactorily.The damage was assessed from histological preparations taken through the site of lesion.Since the dorsal funiculi are not essential for maintaining lactation, there are no obviousdifferences which could explain the difference in lactational ability in terms of an undamagedipsilateral pathway (see text).

degeneration anywhere in the neuraxis. Those of the remainder (P1. 1) showed theexpected ascending and descending degeneration in the spinal cord. The formercould be traced to the gracile and cuneate nuclei in tissues where the lesions had in-volved the dorsal funiculi (PI. 1, figs. 1, 3), and through the dorsal and ventral spino-cerebellar tracts to the white matter of the cerebellum when the lateral and ventralfuniculi of the cord had been interrupted (PI. 1, figs. 2, 3). Diffuse granulation couldoften be seen in the reticular formation of the lower brain stem which was moreconcentrated in the region occupied by the medial lemniscus, but although spino-thalamic involvement might have been expected in lesions to both the lateral andventral funiculi, no consistent degeneration was ever seen above the level of the pons.The absence of such degeneration was not due to the fact that, in the rats of groups5-7, a considerable time (2-3 months) had lapsed between operation and autopsy,

J. T. Eayr8 and B. M. Baddeleyfor in rats which had undergone hemisection of the spinal cord, and whose tissueshad been prepared at the optimum time of 14 days after operation, the findings weresimilar (see PI. 1, fig. 3).

DISCUSSION

The experimental findings help to elucidate the neural pathways by which lactationis maintained by suckling. It is clear, as would be expected, that the pathway entersthe spinal cord at segmental level by way of the dorsal roots. Its subsequent coursewithin the spinal cord however, although strongly indicated, cannot be definedprecisely.The results for the rats of group 8 (hemisection of the spinal cord combined with

unilateral removal of nipples) show that, although a weak contralateral componentmay be present, the pathway follows a predominantly ipsilateral course; but at thesame time it does not seem that the function of maintaining lactation can be assignedto either of those systems which are classically regarded as 'uncrossed'. In the firstplace it would appear most improbable, both on anatomical and on functionalgrounds, that the spino-cerebellar tracts are implicated; and secondly, the findingsargue against the possibility that the fasciculi gracilis and cuneatus are involved.For instance (i) the interruption of these tracts does not cause a complete arrest oflactation; (ii) the impairment which ensues is to some extent transient, andalthough more severe, is similar in type to that which follows exposure of thespinal cord without apparent damage to nervous tissues; and (iii) similar effectsresult from mild lesions to the lateral funiculi, the dorsal funiculi remaining intact.Hence, although the participation of the fasciculi gracilis and cuneatus cannotentirely be excluded, it is more likely that the effects of lesions to these tracts are theindirect results of non-specific trauma (e.g. that caused by interference with the vas-cular supply of neighbouring structures) rather than to the interruption of specificpathways.

It seems therefore that the pathway must be related to the spino-thalamic/spino-tectal system within which the body is believed to be represented bilaterally(Walker, 1940). The present results do not, however, conform with the generallyaccepted organization of the spino-thalamic projections in so far as, in man, only asmall minority of fibres is thought to be uncrossed. Hence unilateral damage to thesystem would be expected to produce its most severe effects contralaterally whilethe reverse is the case where the fibres responsible for maintaining lactation areconcerned. It is unlikely that this anomaly can be explained by the ipsilateral ascent,for a few segments, of either first or second order fibres, for unilateral lesions in thecervical region were similar in their effect to those made at thoracic level. It wouldtherefore appear either that all the fibres which regulate lactation are contained inthe small uncrossed component, or that the spino-thalamic projections as a wholeare differently organized in the rat from those in man. Evidence in support of thelatter view is provided by the observation that, after hemisection of the cord in therat, a squeak can be elicited by pricking either foot. It is thus clear that thisoperation by no means interrupts the pathway mediating the perception of painfulstimuli, as might be expected from the usual effects of unilateral spino-thalamictractotomy in man.

168

Neural pathway in lactation 169The general position within the spinal cord occupied by the fibres concerned in

lactation may be deduced from a comparison of the site and extent of lesions whicharrest lactation and those which do not. As argued above, the dorsal part of thecord can probably be excluded. Lesions to both the lateral and ventral funiculivary in their effect. When, as shown by histological preparations, the lesion to thelateral columns is small, or occurs predominantly in the dorsal part of these columns,lactation is affected no worse than it is by lesions to the dorsal funiculi. Much thesame considerations apply to injury to the medial part of the ventral funiculi. When,however, lesions to the lateral funiculi are made more deeply, or when those to theventral funiculi are extended laterally, lactation ceases. From such observations itmay be inferred that the pathway is predominantly localized deep in the lateralportion of the cord. The fibres appear to be intermingled with those concerned withthe control of the pelvic viscera, and it is of interest that the lesions which preventlactation in the rat are largely co-extensive with those which interfere with micturi-tion in the cat (Barrington, 1933) and with defaecation in man (Nathan & Smith,1953). It might be expected, by analogy with the topical arrangement of fibresin the human spino-thalamic tract proposed by Walker (1940), that fibres mediatingsensation from the abdominal wall would be localized rather more superficially atthoracic and cervical levels than is suggested by the results of the present experi-ments. In the rat, however, the cortico-spinal tracts are carried in the dorsal funi-culi, and the consequent difference in the organization of the lateral columns betweenthe rat and man may account for the apparent discrepancy.

Little information concerning the localization in the brain stem of the pathwaysubserving lactation could be obtained from the study of histological material. Infact, from the absence of any consistent degeneration in Marchi preparations of themesencephalon, even in the tissues of animals killed at the optimum time of 14 daysafter operation, it is clear that the Marchi technique is unsuitable for studying thecourse of this pathway in the rat. It would appear either that (i) the fibres areunmyelinated, (ii) they are not fasciculated in any discretely organized tract butare widely dispersed throughout the reticular formation, or (iii) that the pathwayconsists of multiple relays, in which connexion attention has already been drawn tothe small number of spino-thalamic fibres which reach the thalamus direct in pri-mates (Walker, 1940). An examination of the effects of small electrolytic lesionsdiscretely placed throughout the neuraxis will probably yield more satisfactory in-formation concerning the course of this pathway than will degeneration studies.Work on these lines which, it is hoped, will help to elucidate both the mode oftermination and function of the ascending pathways now studied at lower levels,is already in progress. Incidental observations already made during the presentexperiments suggest that these pathways influence the activity both of the adeno-and of the neuro-hypophysis. For example, the oxytocic principle of the posteriorpituitary, a hormone capable of maintaining lactation after lesions to the supra-optico hypophysial tract (Cross & Harris, 1952), section of the pituitary stalk (Harris& Jacobsohn, 1952) or when given together with anterior lobe extracts after hypo-physectomy (Gomez, 1940), will restore lactation in rats after it has ceased as aresult of lesions made to the spinal cord. Injections of this hormone have beeneffective, however, only for a period of up to 48 hr., after which lactation ceased

170 J. T. Eayrs and R. M. Baddeleyaltogether. It may tentatively be postulated from such findings that both lobes ofthe pituitary cease to exert their characteristic influence over lactation (see Folley,1947) when the arrival of stimuli originating in the nipples is interrupted. Thesecretions of the neuro-hypophysis, which are responsible for the ejection of milk,can be replaced by exogenous oxytocin, but no milk is secreted once the galacto-poietic principles of the adeno-hypophysis have been exhausted.A further observation, which may prove to be of considerable importance for

understanding the neural mechanisms in lactation, is that the return of the abilityto raise young coincided with the re-establishment of sensitivity in the region of theinguinal nipples. Since the sensory innervation of this region was interrupted cen-tral to the dorsal root ganglion, it follows that the return of sensation must have beendue to the ingrowth of nerve fibres from segments below the level of the lesion whichwould not normally be concerned with lactation. If more fully substantiated thisfinding would suggest that the stimulus of suckling may maintain lactation not somuch through an anatomically discrete pathway as by setting up a specific rhythmof discharge determined by an interaction between (i) the form of stimulus, (ii) theproperties of the receptors, and (iii) excitable relationships at central synapses. Sucha possibility has already been suggested by studies on the mechanism underlyingcutaneous sensibility to temperature change (Lele, Weddell & Williams, 1954), amodality which is also mediated by the spino-thalamic system.

SUMMARY

1. A study has been made of the nervous pathways through which lactation ismaintained by the act of suckling.

2. Lactation, as measured by the growth of the litter, is slightly impaired as aresult of exposing the spinal cord, and rather more severely when the dorsal columnsare cut. In both these instances, however, there is a recovery in lactational perform-ance though not to the preoperative level.

3. Lactation is also impaired by mild bilateral lesions to the lateral funiculi of thecord and is inhibited by more severe lesions in this region. It is also prevented bycutting the dorsal roots of the nerves supplying the segments in which the sucklednipples are situated.

4. With few exceptions lactation ceases following hemisection of the spinal cordwhen the only nipples available for suckling are on the same side as the lesion.Hemisection does not cause arrest of lactation when nipples are suckled on the sideopposite to that on which the lesion is made.

5. It has not been possible to trace the course of the pathways interrupted bylesions to the spinal cord from a study of Marchi preparations.

6. It is inferred from these findings that the pathway by which the sucklingstimulus maintains lactation enters the central nervous system by the dorsal roots andascends in the spinal cord deep in the lateral funiculus of the same side. This path-way to the diencephalon is either indirect, unmyelinated, or not fasciculated butwidely dispersed throughout the reticular formation.

Journal of Anatomy, Vol. 90, Part 2

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Neural pathways in lactation 171

REFERENCESBACQ, Z. M. (1932). The effect of sympathectomy on sexual functions, lactation and the maternal

behaviour of the albino rat. Amer. J. Physiol. 99, 444 453.BARRINGTON, F. J. F. (1933). The localization of the paths subserving micturition in the spinal cord

of the cat. Brain, 56, 126-148.CROSS, B. A. (1955). The posterior pituitary gland in relation to reproduction and lactation. Brit.

Med. Bull. 11, 151-155.CROSS, B. A. & HARRIS, G. W. (1952). The role of the neurohypophysis in the milk-ejection reflex.

J. Endocrin. 8, 148-161.EAYRS, J. T. & BADDELEY, R. M. (1955). Le system nerveux et la lactation. Resumes des Com-

munications. VI Congrgs Fgderatif International d'Anatomie, Paris.FOLLEY, S. J. (1947). The nervous system and lactation. Brit. Med. Bull. 5, 142-148.GOMEZ, E. T. (1940). Effect of post-hypophyseal extract on lactation in hypophysectomized post-

gravid rats. J. Dairy Res. 23, 537-538.HARRIS, G. W. & JACOBSOHN, D. (1952). Functional grafts of the anterior pituitary gland. Proc.

Roy. Soc. B, 139, 263-276.HOOKER, C. W. & WILLIAMS, W. L. (1940). Retardation of mammary involution in the mouse by

inhibition of the nipples. Yale. J. Biol. 12, 559-564.INGELBRECHT, P. (1935). Influence du systkme nerveux central sur la mammelle lactante chez le

rat blanco. C.R. Soc. Biol., Paris, 120, 1369-1371.LELE, P. P., WEDDELL, G. & WILLIAMS, C. M. (1954). The relationship between heat transfer, skin

temperature and cutaneous sensibility. J. Physiol. 126, 245-256.NATHAN, P. W. & SMITH, M. C. (1953). Spinal pathways subserving defaecation and sensation from

the lower bowel. J. Neurol. Psychiat. 16, 245-256.PETERSEN, W. E. & RIGOR, T. V. (1932). Relation of pressure to rate and quality of milk secreted.

Proc. Soc. Exp. Biol., N.Y., 30, 254-256.ROMANES, G. H. (1950). The staining of nerve fibres in paraffin sections with silver. J. Anat., Lond.,

84, 104-115.SELYE, H. & McKEOWN, T. (1934). Further studies on the influence of suckling. Anat. Rec. 60,

323-332.SWANK, R. L. & DAVENPORT, H. A. (1935). Chlorate-osmic-formalin method for staining degenerat-

ing myelin. Stain Tech. 10, 87-90.WALKER, A. E. (1940). The spinothalamic tract in man. Arch. Neurol. Psychiat., Chicago, 43,

284-298.

EXPLANATION OF PLATEExtent of degeneration in spinal cord and brain stem following various lesions to the spinal cord.

A: below level of lesion; B-E: in ascending order above level of lesion.Fig. 1. Degeneration following section of dorsal and ventral funiculi. The preparation was made

3 months after the lesions, and much of the original granulation has presumably cleared up.Ascending degeneration in the medulla is confined to the fasciculi and nuclei gracilis (f.g. andn.g.) and to the spino-cerebellar tracts (t.s.c.).

Fig. 2. Degeneration following combined section of dorsal and ventral funiculi (lesion 3 monthsold). The only marked degeneration is in the spino-cerebellar tracts (t.s.c.) in the medullaand in the ventral spino-cerebellar tracts (t.sc.v.), restiform body (c.r.) and white matter of thecerebellum at the pontine level. None can be seen in the mid-brain.

Fig. 3. Degeneration following hemisection of the spinal cord. Preparation made 14 days afterlesion, but even so degeneration is confined to the dorsal funiculi and spino-cerebellar tracts(t.sc.d. and t.sc.v.). The slight unilateral granulation in the medial lemniscus (l.m.) at the levelof the mid-brain was not found consistently.