THE CIRCULATORY SYSTEM 87 



ment to the organ, the spleen is surrounded by 

 a fibrous and muscular capsule, from which 

 sparse trabeculae are continued into the interior 

 to form a coarse net-like fibrous skeleton. A 

 very delicate reticulum, running throughout 

 the whole organ, contains spleen-pulp within 

 its meshes. Collections of lymphoid tissue are 

 associated with the smaller branches of the 

 splenic artery. The walls of the terminal 

 branches of the artery become defective and 

 thus permit the escape of blood into the spleen- 

 pulp, which is consequently largely composed 

 of blood corpuscles. 



The thyroid gland is composed of two lateral 

 lobes which may be found internal to the 

 jugular vein in the angle formed by the diver- 

 gence of the subclavian and common carotid 

 arteries. The thyroid consists of microscopic 

 closed vesicles lined by epithelium and con- 

 taining a homogeneous colloid material. 



The thymus is only present at its maximum 

 in young chicks, where it is in the form of a 

 lobulated body extending along the entire 

 length of the neck. It diminishes in size with 

 age, and in old birds may be entirely absent. 



The adrenals in the adult are oval or 

 elongated bodies (from 8 to 10 mm. long) lying 



