THE VASCULAR SYSTEM. 155 



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

 are sometimes connected by a small isthmus extending ven- 

 trally 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 thyroid unless 

 it is fed thyroids or an extract of thyroid gland. The 

 function of the gland is to supply iodin compounds 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 en- 

 tirely absent in aged specimens. Its function is unknown. 

 It persists throughout 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. 

 Their function is not known. They are sometimes spoken 

 of as suprarenal capsules or adrenal bodies. 



The above-named ductless glands occur in all mammals 

 in the same relative locations. The anatomy of the lym- 

 phatic system is also exceedingly similar throughout the 

 various orders of mammals. 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 centimeters long is usually 

 present. A failure of the lymphatics to do their work 

 results in dropsy. 



PRACTICAL QUESTIONS AND SUGGESTIONS. 



1. Which of the four parts of the lymphatic system are visible in 

 your specimen ? 



2. Describe size and number of lymphatic glands in the mesentery. 



3. Describe the location of lymph glands present in any other portion 

 of your specimen. 



