416 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



of Langerhans are not made up of uniform cells : there are two kinds, 

 one of which is certainly the type that is chiefly concerned with the 

 elaboration of insulin, and it is this type particularly that increases in 

 numbers in the hibernating mammal. 



The islets of the pancreas are, however, by no means the only endo- 

 crine glands that undergo a cyclic change during hibernation. The 

 thyroid gland in the neck, the cortex of the adrenal glands near the 

 kidneys, and the anterior lobe of the pituitary at the base of the brain, 

 all participate in the changes. Of these the anterior pituitary prob- 

 ably takes first place in importance — it has been not inaptly called 

 the "conductor of the endocrine orchestra" because its secretions have 

 such a profound effect on the activities of all the other endocrine 

 glands. The genital glands, which have an endocrine function as well 

 as their primary one of producing gametes, undergo cyclic changes 

 in the hibernators, as in other animals, but although they are depend- 

 ent upon the activity of the anterior pituitary it is doubtful whether 

 they play any important part in the phenomenon of hibernation. 



All these glands are at their lowast phase of activity in the autumn, 

 but during the winter there is a gradual recovery and in the spring 

 they show a great spurt of development that is closely connected with 

 the onset of the breeding season. As far as the thyroid is concerned 

 this burst of activity is short; it is probable that increased tempera- 

 ture during the summer, and the great development of reproductive 

 activity, are responsible for the regression that the gland undergoes 

 from quite early in the spring. The small related glands, the para- 

 thyroids, undergo a parallel cyclic series of changes, but it is probable 

 that they do not undergo any marked functional deficiency during 

 winter. The adrenal is a gland of particular interest and many func- 

 tions; its outer part or cortex contains a zone (the X zone) that is 

 very greatly developed in hibernating mammals during the autumn 

 and winter until the spring, but becomes so reduced as to be practically 

 vestigial during the summer. The central part of the adrenal, the 

 medulla, also undergoes considerable changes during the year, but 

 no marked cyclic activity that can be correlated with hibernation has 

 been identified. The activity of all these glands takes part in the 

 thermoregulation of the body, but the exact role of each remains yet 

 to be elucidated. It may not be out of place, however, to point out 

 that the "anterior pituitary-thyroid-adrenal axis" is closely con- 

 cerned with the peculiar "shock disease" that sometimes aJG3icts whole 

 populations of animals, particularly those of rodents that periodically 

 build up a peak of numbers far greater than the territory that they 

 inhabit can support. 



A last point : It has recently been shown that in some hibernating 

 mammals the rate of clotting of the blood when shed is greatly re- 



