160 THE LYMPHATIC SYSTEM 



to the lobule which penetrate to the medulla, where the}' form a 

 plexus of sinusoidal capillaries with elongated meshes, and also 

 distribute radiating capillaries to the cortical portion of the lobule. 

 These sinusoidal vessels are highly characteristic of the medulla 

 of the thymus lobule. They unite to form venous radicals of con- 

 siderable caliber, which leave the lobule to join the interlobular 

 veins in the loose connective tissue between the lobules. 



Lymphatic vessels are of frequent occurrence in the interlob- 

 ular connective tissue, and their branches occasionally penetrate to 

 the medulla of the lobule. Small nerve trunks are also found in 

 the interlobular connective tissue, but seem to be chiefly distrib- 

 uted to the walls of the larger blood vessels. 



THE SPLEEN. The spleen contains a pulp which closely 

 resembles adenoid tissue, and is surrounded by a well-developed, 

 fibro-muscular capsule. The spleen pulp, as indeed the entire 

 structure of the organ, is intimately related to its blood supply. 



The framework of the organ includes a capsule, numerous tra- 

 beculae, and a delicate reticulum. The spleen is also inclosed by 

 a reflection of the peritoneum which supplies a serous coat to all 

 portions of the surface of the organ, except at the attachments of 

 the gastro-splenic omentum and the phreno-splenic and lieno-renal 

 ligaments. 



The capsule of the organ, to which its serous coat is loosely 

 attached, comprises two layers, an outer fibrous and an inner mus- 

 cular. In some animals e. g., the ox these layers are much 

 more highly developed than in man. The outer layer consists of 

 dense interlacing bundles of fibrous tissue, in which is an abun- 

 dant network of elastic fibres. The inner layer contains, in addi- 

 tion to the fibro-elastic membrane, a considerable amount of smooth 

 muscle which forms interlacing bundles. In the ox, in which ani- 

 mal this coat is most highly developed, two layers, the fibres of 

 which cross each other at right angles, may be distinguished in 

 the muscular portion of the capsule. 



From the inner surface of the capsule trabeculse extend into 

 the interior of the organ and penetrate to all its portions. These 

 trabeculae consist of fibrous and elastic tissue, with which bundles 

 of smooth muscle fibres are intermingled. At the hilum a large 

 mass of trabecular tissue is carried into the interior of the organ 

 along with the larger blood vessels. Those trabeculae which 

 arise in this manner serve as sheaths for the larger arteries and 

 veins. 



