534 VERTEBRATE LIFE AND ORGANIZATION 



of evidence that the placenta may produce hydrocortisone, cortisone 

 and other adrenal corticoids, and a hormone similar to ACTH. 



In some animals, such as the rabbit, the placenta is a significant 

 source of relaxin. This protein hormone, also produced by the ovary, 

 functions to relax the ligaments of the pelvis to facilitate the birth of 

 the young. Relaxin is effective only after the connective tissue of the 

 pubic symphysis has been sensitized by the action of estradiol. Relaxin 

 also inhibits the motility of the uterine muscles. 



The production of estrogens and progesterone, as reflected by the 

 amount present in blood and urine, increases gradually throughout 

 pregnancy, reaches a peak just before or at the time of parturition, then 

 abruptly declines after birth (Fig. 30.14). The factors which determine 

 the onset of labor, the expulsion of the fetus from the uterus, remain a 

 mystery. The possibility that oxytocin has a role in this was mentioned 

 (p. 622). There are many hormonal changes which occur at about the 

 time of parturition— decreases in estrogen and progesterone, and an in- 

 crease in chorionic gonadotropin— but whether these are causes, effects, 

 or unrelated phenomena remains to be determined. 



258. Other Endocrine Glands 



The thymus and pineal body may have endocrine functions. The 

 thymus lies in the upper part of the chest, just above the heart. Its cells 

 closely resemble lymph tissue. The thymus is large during the years of 

 rapid growth but begins to regress after puberty. It has been postulated 

 to affect growth or sexual maturity, but extirpation of the gland or the 

 administration of extracts fails to reveal any endocrine function. The 

 pineal body is a dorsal outgrowth of the diencephalon which lies on the 

 upper surface of the thalamus. It has been suspected of having some role 

 in body growth and genital development, but the evidence is somewhat 

 conflicting and no endocrine function can be ascribed to it with cer- 

 tainty. 



The cells of certain parts of the digestive tract are known to secrete 

 hormones in response to the presence of certain kinds of food, which 

 stimulate the production and release of digestive juices. These are sum- 

 marized in Table 9. 



Table 9. HORMONES OF THE DIGESTIVE TRACT 



STIMULUS FOR RESPONSE OF 



HORMONE SECRETED BY TARGET ORGAN 



SECRETION TARGET ORGAN 



®°**"" Pyloric mucosa Presence of food Mucosa of Secretion of 



in stomach stomach fundus gastric juice 



Secretin Duodenal mucosa Presence of acid Pancreas Secretion of 



food in duo- pancreatic juice 



denum 

 Enterogastrone Duodenal mucosa Neutral fat Stomach Decreased motility 



and secretion of 

 HCl 

 Cholecyslokinin Duodenal mucosa Acid food Gall bladder Contraction of 



gall bladder 



