1326 



HORMONAL REGULATION OF BEHAVIOR 



in association with the onset of lactation 

 (Reece and Turner, 1937a). In the rabbit, 

 this initial rise in pituitary prolactin, from 

 about 12 pigeon units per gland to about 

 23 pigeon units, occurs between the 28th 

 day of pregnancy and the 2nd day postpar- 

 tum. The rise is the same in suckled and 

 nonsuckled rabbits. However, after the 2nd 

 day postpartum, suckled rabbits have more 

 prolactin in their pituitary glands, and pro- 

 duce more milk from the mammary gland, 

 than nonsuckled animals (Meites and Tur- 

 ner, 1942c; Meites, 1954). Similar data 

 have been obtained from the rat (Meites 

 and Turner, 1948) . 



In the dairy cow, which is of course 

 highly selected for milk production, lacta- 

 tion can sometimes be started in nonpreg- 

 nant or even virgin cows by regular manip- 

 ulation of the teat. In general, however, it 

 is clear that suckling stimuli are not effec- 

 tive in inducing lactation, and that the ini- 

 tiation of lactation, as distinct from its 

 maintenance, must be explained on the ba- 

 sis of the hormonal changes occurring at 

 the time of parturition. 



The nature of the suckling stimulus. 

 Selye, Collip and Thomson (1934) removed 

 the nipples of the mammary glands on one 

 side of a group of rats and tied off the main 

 milk ducts on the other side. When lactat- 

 ing animals so treated were kept with their 

 young, no involution of the milk-secreting 

 tissue occurred on either side, indicating 

 that the effect of the suckling stimulus on 

 the mammary glands was through a sys- 

 temic relationship, rather than being local 

 to the tissues in the neighborhood of the 

 stimulation. Ingelbrecht (1935) later dem- 

 onstrated the same point in an ingenious 

 group of experiments. He sectioned the 

 spinal cord of 10 rats 2 days postpartum 

 between the last thoracic and first abdomi- 

 nal segments, thus ensuring that the 6 pos- 

 terior nipples were anesthetized, whereas 

 the 6 anterior nipples retained their sensi- 

 tivity to stimulation. If the anterior (un- 

 anesthetized) nipples were covered, so that 

 the young could only suckle from the pos- 

 terior nipples, all the young died within 48 

 hours, with empty stomachs. Replacement 

 litters suffered the same fate. If only 4 of 

 the unanesthetized nipples were covered, 

 leaving 2 unanesthetized nipples free for 



access by the suckling young, then all parts 

 of the mammary glands continued to se- 

 crete milk. After two or three groups of 

 young died while attempting to suckle on 

 the posterior nipples, the anterior nipples 

 were uncovered, and a new litter presented 

 to the experimental mothers. They sur- 

 vived, and lactation was maintained 

 throughout the mammary gland tissue. 

 Eayrs and Baddeley (1956) repeated, in es- 

 sence, the experiment of Ingelbrecht, ex- 

 cept that they anesthetized some of the nip- 

 ples by cutting the dorsal roots of their 

 spinal nerves, rather than by sectioning the 

 spinal cord, and then removed the unanes- 

 thetized nipples, whereupon lactation 

 stopped altogether, although the remaining 

 (anesthetized) nipples were vigorously 

 suckled. Lactation was re-established in 

 later broods, the re-establishment of the 

 ability of stimulation of the nipples to in- 

 duce lactation corresponding with the re- 

 generation of tactual sensitivity in the 

 nipple area. Ernst (1929) showed that de- 

 nervation of the mammary glands of a dog 

 resulted in regression of the milk-secreting 

 tissue. 



Hooker and Williams (1940) and Mixner 

 and Turner (1941) stimulated the nipples 

 of lactating mice, whose young had been re- 

 moved, by applying turpentine on the nip- 

 ple surface. Both groups of experimenters 

 found that this treatment retarded the in- 

 volution of the mammary gland which nor- 

 mally follows the removal of the young. 

 Mixner and Turner suggested that the tur- 

 pentine acted by inducing hyperemia lo- 

 cally, but Hooker and Williams' observa- 

 tion that, when turpentine was applied only 

 to some of the nipples, involution was pre- 

 vented in all the mammae indicates that 

 the action of turpentine is not local, but is 

 through the central effect of the sensory ir- 

 ritation of the nipples. 



These experiments clearly demonstrate 

 that the suckling stimulus consists pri- 

 marily of mechanical and tactual stimuli 

 applied to the nipple, and that its effect is 

 not local on the mammary glands, but 

 through the central effect of the afferent 

 inflow from the nipples. 



The effects of the suckling stimulus. 

 (a) On the pituitary gland. A number of 

 investigators have studied the effect of 



