i6o 



HISTOLOGY. 



in reptilian spleens that they resemble closely mammalian haemolymph 

 glands. In the guinea pig the lymphoid sheath of the arteries is continuous, 

 though narrow; in man it is so interrupted as to form a succession of spindle- 

 shaped or spherical masses called splenic nodules [Malpighian corpuscles]. 

 An arterial branch passes through each nodule. Thus, as compared with 

 the haemolymph gland, the spleen is deficient in lymphoid tissue (Fig. 180). 

 The bulk of the spleen is composed of splenic pulp, which corresponds 

 with the blood sinus of the haemolymph glands. Its framework of retic- 

 ular tissue is continuous with that of the nodules, and it contains blood cor- 

 puscles of all sorts, special phagocytic cells known as splenic cells, and the 

 terminal branches of both arteries and veins. There are no lymphatic 

 vessels within the spleen. The capsule and trabecular framework are 

 highly developed as in the largest lymph glands. The following features 



sp.n 



art 



PIG. 180. DIAGRAM OP A HAEMOLYMPH GLAND, A, AND OF A PART OF THE SPLEEN, B. 

 The arteries are shown as slender lines (art.) and the veins as heavy ones (v.) ; c., capsule ; b. s., blood 

 sinus, corresponding with the splenic pulp, p.; s. n., secondary nodule; sp. n., splenic nodule; tr., 

 trabecula. 



of the spleen may be described in turn; the blood vessels, the pulp, the 

 nodules, the capsule and trabeculae, and finally the nerves. 



As shown in the diagram, Fig. 181, the splenic artery enters at the 

 hilus and, accompanied by veins, its branches are found in the largest tra- 

 beculae. When about 0.2 mm. in diameter the arteries leave the trabec- 

 ulae in which the veins continue further. The arteries, however, are still 

 surrounded by a considerable connective tissue layer, the outer portion of 

 which becomes reticular and filled with the lymphocytes of the nodules. 

 The nodules occur near where the artery branches. Small arterial twigs 

 ramify in the nodules, in the periphery of which they anastomose before 

 passing on to the pulp. When the main stems are about 15 /.i in diameter 

 they lose their surrounding lymphoid layer and pass into the pulp where 

 they form brush-like groups of branches (penicilli}. These branches do 



