THE SPLEEN. 



953 



of the body. The cells which it encloses, like the supporting cells of the pulp, are 

 possessed of amoeboid movements, but when treated with carmine become deeply 

 stained, and can thus easily be recognized from those of the pulp. 



The arterioles terminate in capillaries, which traverse the pulp in all directions : 



Supporting cell. 



Vessel undergoing lymphoid change. 



Small 

 artery.' 



Vessel continuous 

 with processes of 

 supporting cells. 



Supporting cell. 



I 



FIG. 526. Section of spleen, showing the termination of the small blood-vessels. 



their walls become much attenuated, lose their tubular character, and the cells of 

 the lymphoid tissue of which they are composed become altered, presenting a 

 branched appearance and acquiring processes which are directly connected with 

 the processes of the sustentacular cells of the pulp (Fig. 526). In this manner the 

 capillary vessels terminate, and the blood flowing through them finds its way into 

 the interstices of the reticulated tissue formed by the branched connective-tissue 

 corpuscles of the splenic pulp. Thus the blood passing through the spleen is 

 brought into intimate relation with the elements of the pulp, and no doubt under- 

 goes important changes. 



After these changes have taken place the blood is collected from the interstices 

 of the tissue by the rootlets of the veins, which commence much in the same way 

 as the arteries terminate. Where a vein is about to commence the connective- 

 tissue corpuscles of the pulp arrange themselves in rows in such a way as to form 

 an elongated space or sinus. They become changed in shape, being elongated 

 and spindle-shaped, and overlap each other at their extremities. They thus form 

 a sort of endothelial lining of the path or sinus, which is the radicle of a vein. 

 On the outer surface of these cells arc seen delicate transverse lines or markings 

 which are due to minute elastic fibrillge arranged in a circular manner around the 

 sinus. Thus the channel obtains a continuous external investment, and gradually 

 becomes converted into a small vein, which after a time presents a coat of 

 ordinary connective tissue, lined by a layer of fusiform epithelial cells which are 

 continuous with the supporting cells of the pulp. The smaller veins unite to form 

 larger ones which do not accompany the arteries, but soon enter the trabecular 

 sheaths of the capsule, and by their junction form from six or more branches which 

 emerge from the hilum and, uniting, form the splenic vein, the largest radicle of 

 the vena porta. 



The veins are remarkable for their numerous anastomoses, while the arteries 

 hardly anastomose at all. 



The lymphatics originate in two ways, i. e., from the sheaths of the arteries 

 and in the trabeculse. The former accompany the blood-vessels, the latter pass to 

 the superficial lymphatic plexus, which may be seen on the surface of the organ. 

 The two sets communicate in the interior of the organ. They pass through the 

 lymphatic glands at the hilum, and terminate in the thoracic duct. 



