98 ELEMENTS OF HISTOLOGY. [Chap. xin. 



cortex than in the medulla, and they radiate from the 

 periphery towards the central parts. 



129. After birth, the thymus begins to undergo 

 degeneration, leading to the gradual disappearance 

 of the greater portion of the gland, its place being 

 taken by connective tissue and fat. But the time 

 when the involution is completed varies within very 

 wide limits. 



It is not unusual to find in persons of fifteen or 

 twenty years of age an appreciable amount of thymus 

 tissue. In some animals, e.g., guinea-pigs, the invo- 

 lution of the gland even in the adult has not made 

 much progress. In the thymus of the dog Watney 

 found cysts lined with ciliated epithelial cells. 



CHAPTER XIII. 



COMPOUND LYMPHATIC GLANDS. 



130. THE compound or true lymphatic glands are 

 directly interpolated in the course of lymphatic vessels. 

 Such are the mesenteric, portal, bronchial, splenic, 

 sternal, cervical, cubital, popliteal, inguinal, lumbar, 

 glands. Afferent lymphatic vessels anastomosing into 

 a plexus open at one side (in the outer capsule) into 

 the lymphatic gland, and at the other (the hilum) 

 emerge from it as a plexus of -efferent lymphatic tubes. 



131. Each true lymphatic gland is enveloped in a 

 fibrous capsule which is connected with the interior and 



the hilum by trabeculce and septa of connective tissue. 

 The trabeculai having advanced a certain distance, about 

 one- third or one-fourth, towards the centre, branch into 

 minor trabeculse, which in the middle of the gland anasto- 

 mose with one another so as to form a plexus with small 



