248 The Vertebrate Organ Systems 



termination of pregnancy. In this manner it is often employed to 

 accentuate uterine contractions and to facilitate expulsion of the fetus. 



The Function of Pitressin. — This hormone has two effects in 

 the normal organism. One important influence is on blood pressure. 

 Pitressin affects the constriction of the smooth musculature of the 

 blood vessels and thus elevates blood pressure when administered. 

 The second important effect is on kidney function where it affects the 

 reabsorption of water by the kidney tubules. 



The Composition of Pituitrin. — Both pitocin and pitressin are 

 protein in nature. 



The Effect of Hypoactivity. — So far as is known there is but one 

 pronounced effect of the hyposecretion of the posterior lobe. This re- 

 sults in the disease, diabetes insipidus, which is characterized by ex- 

 cessive fluids loss through the kidneys. Large amounts of urine of a 

 low specific gravity and free of sugar are voided. Relief of this condi- 

 tion is afforded by the administration of pitressin. 



THE DIGESTIVE HORMONES 



In the previous discussion on the digestion of foods (Fig. 43), it 

 was pointed out that the secretion of some of the digestive enzymes 

 was initiated or maintained by certain specific hormones. 



Briefly these are as follows : 



Gastrin, secreted by the cells of the stomach wall, stimulates further 

 secretion by the gastric glands. 



The remaining hormones are secreted by the wall of the small 

 intestine : 



Enterokinase activates the enzyme trypsinogen. 



Secretin stimulates the flow of water and bicarbonates from the 

 pancreas, while pancreozymin stimulates the secretion of the pancre- 

 atic enzymes. 



The emptying of the gall bladder is stimulated by cholecystokinin, 

 while secretin is apparently able to increase the flow of bile. 



Another hormone, enterogastrone, affects the stomach, causing an 

 inhibition of the flow of gastric juice and movements of the muscles 

 of the stomach. 



Finally it is possible that the initiation and secretion of the intes- 

 tinal juices is affected by another hormone, enterocrinin, which is lib- 

 erated after the acidic chyme enters the intestine from the stomach. 



