318 HISTORY OF THE HUMAN BODY 



certain localized portions of the system of tubes, the walls of 

 which are caused to dilate and contract rhythmically through 

 the development of a layer of involuntary muscles. 



Vertebrates possess the tubular or closed type of vascular 

 system, reinforced by a few definitely localized lacuna, and 

 indirectly aided by the various serous cavities of the body like 

 the ccelom and the capsules of the joints. Both anatomically 

 and physiologically this system is divided into two subordi- 

 nate systems, hccmal and lymphatic, of which the first is the 

 one principally emphasized, while the other bears to it the 

 relationship of an important auxiliary. The tubules of the 

 first system are divided into the heart, a localized pulsating 

 vessel with enormously hypertrophied muscular walls ; arteries, 

 in which the current flows from the heart ; veins, in which the 

 current flows toward the heart ; and lastly capillaries and sinu- 

 soids, two forms of the minute vessels which extend between 

 the arteries and veins and supply every tissue of the body. To 

 these, which collectively bear the name of blood-vessels, there 

 are associated a few definitely bounded lacuna, here spaces 

 limited by membranes, and mainly differing from the rest of 

 the system, into which they are continued, by the absence of 

 walls of their own. The circulatory medium contained in 

 this system is termed blood, and consists of two main types 

 of cells, the erythrocytes or " red blood corpuscles," and the 

 leucocytes or " white blood corpuscles," suspended in a 

 liquid plasma. 



The auxiliary system consists primarily of lymphatic vessels, 

 which in distinction from the veins and arteries are small 

 and thin-walled, and of lymph glands, which are not glands 

 in the usual sense, but storehouses for leucocytes. With the 

 lymphatic system are associated the serous cavities of the 

 body (ccelom, capsules of joints, bursse about the larger ten- 

 dons, etc.), with which numerous lymphatic vessels communi- 

 cate so that, by a physiological though not a morphological 

 right, these cavities have been considered as expanded 

 lymphatic vessels. In the lower vertebrates a number of 

 definitely located pulsating organs, or lymph hearts, further 



