THE FROG : VISCERA AND VASCULAR SYSTEM 51 



when it is to be transferred to other parts of the body, 

 is converted into sugar and cast into the blood. In the 

 liver also the nitrogenous waste is converted into urea ready 

 for excretion by the kidneys, and various other chemical 

 changes take place. 



We must here mention certain organs known as the 

 ductless glands, which, while they manufacture 

 clanlfs! 8 substances of importance to the body, 1 dis- 



charge these products not through a duct but 

 into the blood or lymph by a process known as internal 

 secretion. A similar process takes place in other organs — 

 for instance, as we have seen, in the liver. 



The thyroid glands of the frog are a pair of small, rounded, pinkish 

 bodies lying on the external jugular veins. Their secretion has an 

 important, but not well understood, action in maintaining the normal 

 working of various parts of the body. 



The adrenal bodies (so-called suprarenal glands) are small yellowish 

 masses lying on the ventral surface of the kidneys. They secrete a 

 substance which affects the ' ' tone " of involuntary muscle. 



The thymus is a small body which lies behind and above the angle 

 of the jaw on each side. Its functions are unknown. 



The pituitary body or hypophysis lies in the skull below the brain 

 (see p. 72). Its presence is essential for life, but its mode of action 

 is not understood. It appears, among other functions, to have an 

 influence on growth. 



The internal secretion of the ductless glands, as well as some of that of 

 other organs, is an instance of the formation of hormones or " chemical 

 messengers, " small quantities of which, passed into the blood, produce 

 effects in distant organs. These substances bring about a chemical co- 

 ordination of the activities of the organism, side by side with that which 

 is accomplished through the nervous system (see p. 76). Thus the 

 production at the right moment of large quantities of gastric and pan- 

 creatic juice is brought about by the stimulation of the glands, partly by 

 impulses through the nervous system during feeding, and partly by 

 hormones, known as secretins, formed in the wall of the alimentary 

 canal under the influence of substances in the food and carried by the 

 blood to the glands. 



The spleen is a small, round, dark red body, lying in the 

 mesentery opposite to the beginning of the rectum. It is 

 engaged in the manufacture of the little cells which float in 

 the blood and are called " blood corpuscles " (see p. 99), and 

 perhaps also in the destruction of some of them. It is also 



1 It is possible that some of these glands are of importance also in 

 destroying poisonous substances formed in the course of metabolism. 



