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. 
