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 gland commences to 

 undergo degeneration, leading to the gradual dis- 

 appearance 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 broad limits. 



It is not unusual to find in individuals of fifteen 

 to twenty y ^ars of age still an appreciable amount of 

 thymus gland tissue. In some animals e.g., guinea- 

 pigs the involution 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. 



1 30. 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, 

 &c., 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 connective tissue trabeculce and septa. 

 The trabeculse having advanced a certain distance, 

 about one-third or one-fourth, in a manner more or 

 less radiating towards the centre, branch into minor 

 trabeculae, which in the middle part of the gland anasto- 

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



