THE DUCTLESS GLANDS 1335 



THE SPLEEN 



This organ is similar in many respects to a lymphatic gland. It is 

 formed of a framework of connective tissue and unstriated muscular 

 fibres, in the interstices of which is contained the splenic pulp. This 

 consists of a fine fibrillar network, on the fibrils of which lie endo- 

 thelial cells. The meshes contain the cells of the splenic pulp, which 

 are fairly large polygonal cells, and leucocytes. Just as in a lymphatic 

 gland the cellular elements of the tissues are bathed by the lymph 

 which flows through the gland, so in the spleen the walls of the capil- 

 laries become discontinuous, and the blood is poured out into the 

 interstices of the tissue. The spleen is therefore the only tissue in the 

 body where the blood comes in actual contact with the tissue-elements 



PRESSURE. 



FIG. 549. Plethysmographic tracing of spleen (upper curve) from a dog, showing 

 the spontaneous contractions of this organ (reduced from a tracing by SCHAFER). 



themselves. The blood from the splenic pulp is collected into large 

 venous sinuses, which run along the trabeculse to the hilum, where 

 they unite to form the splenic vein. The arteries to the spleen are 

 beset in their course along the trabeculse with small nodules of lymphoid 

 tissue, which are known as the Malpighian follicles. 



It is evident that the blood must meet with considerable resistance 

 in passing through the close meshwork of the splenic pulp. In order 

 to ensure a constant circulation through the gland, the muscular 

 tissue of the capsule and trabeculse has the property of rhythmic 

 contraction. If the spleen be enclosed in a plethysmograph, or splenic 

 oncometer, and its volume be recorded by connecting this with an 

 oncograph, it will be seen to be subject to a series of large, slow 

 variations, each contraction and expansion lasting about a minute, 

 and recurring with great regularity (Fig. 549). Superposed on 

 these large waves are smaller undulations due to the respiratory 

 variations of the blood pressure, and on these again the little excur- 

 sions corresponding to each heart-beat. The contractile power of 

 the spleen is under the control of the nervous system, and a rapid 



