The Endocrine System 247 



only after the mammary gland has first been prepared by estrogen and 

 progesterone. It is generally assumed that the action of estrogen and 

 progesterone is antagonistic to that of prolactin ; however, immediately 

 after parturition, the high levels of these two hormones in the body 

 is suddenly lowered, and the prolactin can then take effect. The main- 

 tenance of lactation is in part due to nervous responses as well as to 

 the interaction of other hormones. Prolactin thus is only a contribut- 

 ing factor. 



The Composition of the Secretions of the Anterior Pituitary. 



— All the secretions of this gland are protein in nature ; their exact chem- 

 ical composition remains to be learned. 



The Effect of Hypoactivity. — Considerable discussion of the 

 effects of hypoactivity has already been done. It is, however, impor- 

 tant to point out that most bodily processes are interfered with. Thus 

 an animal that is deprived of its anterior pituitary gland is unable 

 to metabolize carbohydrates properly; fat metabolism also is faulty. 

 Hyposecretion of the various endocrine glands which are in part con- 

 trolled by the pituitary may also result. 



The Effect of Hyperactivity. — Most of these effects also have 

 been indicated above. 



THE INTERMEDIATE LOBE 



Although this portion of the pituitary is present in most vertebrates, 

 its action has been demonstrated only in the fishes, amphibians, and 

 reptiles. In these animals, it secretes a hormone known as intermedin 

 which controls the expansion and contraction of pigment cells in the skin. 



THE POSTERIOR LOBE 



This portion of the pituitary has two recognized hormones : pitocin 

 and pitressin. The secretion containing these two hormones is often 

 called pituitrin. 



The Function of Pitocin. — The best known effects of this hor- 

 mone are its excitation of smooth muscle; more specifically, this hor- 

 mone affects the smooth muscle of the uterus. The response of this 

 smooth muscle is in most mammals first conditioned by the ovarian 

 and placental secretions. It is possible that pitocin is involved in the 



