THE VASCULAR SYSTEM 1 89 



forms blood-corpuscles, but in post-natal life it seems to destroy 

 blood-corpuscles. An animal from which the spleen has been 

 removed may live many years in good health. 



The thyreoid gland is composed of two parts lying on the 

 lateral aspects of the trachea, just caudad to the larynx. Each 

 part is less than two centimeters long. The two parts are 

 sometimes connected by a small isthmus extending ventrally 

 across the trachea, as in man. An enlargement of this gland 

 in man is called goiter. An animal can live only a short time 

 after the removal of the thyreoid unless it is fed thyreoids or 

 an extract of thyreoid gland. The function of the gland is to 

 supply iodin compounds and certain internal secretions to the 



system. 



The thymus gland is a median structure varying in size 

 according to age. In a cat about one-third grown it is very 

 large, extending craniad from the heart along the ventral 

 aspect of the trachea three or four centimeters. As the cat 

 grows older, it gradually dwindles, and is entirely absent in 

 aged specimens. Its function is unknown. It persists through- 

 out life in the lower vertebrates. 



The suprarenal bodies are small, somewhat bean-shaped 

 organs lying craniad of the kidneys. An animal from which 

 they have been removed can live but a short time. They 

 give to the blood an internal secretion very essential to life. 

 A hormone known as adrenalin is one of the products, ''prob- 

 ably having to do with maintaining the proper tonus of the 

 muscle of the heart and blood-vessels thus underlying blood- 

 pressure." They are sometimes spoken of as suprarenal cap- 

 sules or adrenal bodies. 



The above-named ductless glands occur in all mammals in 

 the same relative locations. The anatomy of the lymphatic 

 system is also similar throughout the various orders of mam- 

 mals. In some animals, especially man, the lymphatic glands 

 are more numerous than in the cat. The; tracheal trunks are 

 also wanting in man and a right thoracic duct about two centi- 



