THE DIGESTIVE SYSTEM 111 



blood-vessels are capable of holding a considerable quantity of 

 blood which increases after each meal reaching a maximum at 

 about the fifth hour. Animals live with little inconvenience 

 when it is removed. 



The thyroid gland is composed of two lateral lobes connected 

 by an isthmus. It is situated in the neck below the anterior part 

 of the trachea and may be easily felt in the living horse. It is 

 very vascular, firm in texture, and dark red-brown in color. 

 Enlargement of the gland results in the disease termed "goiter." 

 An important internal secretion containing iodine compounds, 

 which has to do with the process of nutrition, is produced by the 

 thyroid. Animals live but a short time after the entire gland, 

 including the parathyroids, has been removed. If they are fed 

 on fresh thyroid tissue from another animal they seem to suffer 

 no inconvenience. 



The thymus gland is found between the heart and the larynx. 

 In late fetal life and in the very young animal it is seen as a long 

 chain of gland-like tissue extending the entire length of the 

 trachea. It atrophies rapidly after birth, and appears to have 

 some regulating effect in the processes of growth and development 

 of the fetus. As a food it is considered by many a delicacy, and 

 is known as the "neck sweetbread." In animals castrated early 

 it atrophies much slower than in others. 



The adrenals are paired ductless glands situated in contact 

 with each kidney (see Fig. 45). They are about 3 inches long 

 and 1}4 inches wide. A substance known as adrenalin has been 

 extracted from the glands. It contains adrenin or epinephrin 

 which is responsible for the powers of the extract. It exerts a 

 powerful action on the small blood-vessels, contracting them to a 

 marked degree, and causing an enormous rise in the blood pres- 

 sure. The removal of the adrenals leads to rapid death preceded 

 by great prostration and depletion, due to loss of the adrenal 

 function of maintaining muscular and vascular tone. 



The pituitary and pineal bodies he at the base of the brain. 

 Various theories have been advanced to explain their functions. 

 The pituitary body is known to contain substances that exert 

 an influence over blood pressure and metabolism. Experiments 

 have shown that the administration of pituitary extract to 

 lactating animals slightly increased the quantity of milk secreted 

 and also its fat content. The increase is temporary, as the 



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