THE FROG 53 



concentration of substances, muscle tone, blood sugar, etc. Instincts, 

 emotions, mental and psychic states are stimulated, inhibited and com- 

 plicated by endocrine action. 



The two thyroid glands are situated on either side of the hyoid, and 

 secrete quite a quantity of iodin. In man, its atrophy is associated with 

 the disease called cretinism ( ) where certain parts 



of the body, such as the head, may become very large. Cretins are 

 almost always idiots. 



The two thymus glands ( ) lie one behind each 



tympanum. They are small and oval in shape, usually reddish in color, 

 and are placed directly beneath the depressor mandibular muscle. The 

 thymus, like the thyroid, diminishes in size with age. 



The adrenal bodies ( ) are little bands of a 



yellowish color extending along the mid ventral surface of the kidneys. 

 They secrete adrenalin, a substance necessary for the life of the animal. 

 This substance is used to a considerable extent in medicine at the pres- 

 ent time as it will cause a contraction of the blood vessels and raise the 

 blood pressure after injection. However, a little later a reaction sets in, 

 and a lowering of blood pressure follows. 



The spleen ( ) is a reddish organ lying imme- 



diately dorsal to the anterior end of the cloaca. It is supposed to act as 

 a sort of filter for the blood. The old corpuscles are destroyed and new 

 colorless ones are formed. It must be remembered that all that is known 

 in regard to the ductless glands demonstrates that they are of vital im- 

 portance, but that no absolute conclusions can be drawn as to definite 

 functions of any of .them ; for, while one or two of their functions are 

 known, there are probably many more functions that are not yet even 

 dreamed of. 



THE CIRCULATORY SYSTEM 



It is essential that the student grasp the fact that there are several 

 types of circulation. The systemic proper is that closed system of blood 

 vessels by which the blood leaves the heart and passes through the large 

 arteries into the capillaries to carry nourishment to every point in the 

 body (Fig. 9). These arterial capillaries then meet with the venous 

 capillaries, and waste-matter is collected in the blood and carried bv the 

 veins into two anterior and one posterior venae cavae by which the blood 

 is returned to the heart. 



The heart itself, however, must have blood vessels carrying nour- 

 ishment to the heart-walls just as an engine run by steam and supplying 

 water to different parts of a building, must have water supplied to its 

 own boiler in order that the steam which gives the engine its power may 

 continue to be generated. 



The heart muscle, which is the engine of the body, must similarly 

 have its supply of blood to its own walls in order that the heart may 



