1 66 ELEMENTS OF MAMMALIAN ANATOMY. 



prolongations of the enveloping capsule. The two 

 forms of tissue can be seen by cutting the organ trans- 

 versely. The dense lymphoid tissue appears as white 

 spots less than the size of a pin-head. They are the 

 Malpighian corpuscles. The intervening looser lymphoid 

 tissue forms the greater part of the spleen, and is 

 known as the splenic pulp. The splenic artery, a branch 

 of the cceliac axis, enters the spleen at its hilus and 

 divides into capillaries which terminate in irregular 

 spaces, thus permitting the blood to flow freely through 

 the splenic pulp, whence it is taken up by the capil- 

 laries of the splenic vein leading to the portal vein. In 

 embryonic life the spleen forms blood-corpuscles, but in 

 postnatal life it seems to destroy blood-corpuscles. An 

 animal from which the spleen has been removed may 

 live many years in good health. 



The thyroid 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 isth- 

 mus 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 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 cephalad from the heart along 

 the ventral aspect of the trachea three or four centi- 

 meters. As the cat grows older, it gradually dwindles, 

 and is entirely absent in aged specimens. Its function is 

 unknown. It persists throughout life in the lower ver- 

 tebrates. 



