H. ORGANS OF CIRCULATION. 



(VASCULAR SYSTEM.) 



THE organs of circulation consist of a central organ, the heart, 

 peripheral organs, the vessels, and nutritive fluids, composed of 

 plasma and structural elements (cells), 1 the blood and 

 lymph. The latter, which occurs partly within closed canals, 

 partly in various spaces and cavities of the body, and which 

 penetrates all the tissues, will be spoken of later, and the blood- 

 vascular system in its more restricted sense will be treated of 

 first. This consists of a series of completely closed tubes (vessels), 

 which, according as they contain oxygenated or impure blood, are 

 spoken of as arteries or veins. This, however, is not an abso- 

 lute rule, for setting aside the chemical condition of the blood, all 

 vessels which empty their contents into the heart are called veins, 

 while those which arise from the heart are spoken of as arteries. 



The heart, which is enclosed within the pericardium/ 2 serves as 

 the central organ of the circulation, and acts both as a suction- 

 pump and a force-pump. It arises, like the entire vascular 

 system, from the mesoblast, either as a single or as a paired 

 tubular cavity ; it originates in the splanchnic layer along the ventral 



1 The blood- and lymph-corpuscles are the last results of segmentation in the 

 mesoblast. The first to be formed are white corpuscles, which are nucleated and 

 amoeboid (these are the only kind present in Amphioxus) ; the red appear secondarily; 

 whether they originate from the white corpuscles or independently is not known, 

 though the former mode of development seems the most probable. Both primitive 

 red and primitive white corpuscles possess a nucleus, which in the case of the 

 latter persists throughout life, though it is often only visible by means of reagents. 

 In the case of the red corpuscles the nucleus persists, and the whole cell is biconvex 

 in all Vertebrates below Mammals, and, even in these, nucleated red cells may be 

 seen in the marrow of the bones, in the blood of the spleen, and often in that of the 

 portal vein. In all other parts of the body of Mammals they lose their nuclei and 

 become biconcave. In all Mammals, except the Camelidae, the red corpuscles have 

 the form of circular disks ; in the last-mentioned group and in all other Vertebrates 

 except Cyclostomes they are oval. Siren possesses the largest red corpuscles, then 

 comes Proteus, and then Salamandra ; the smallest are found in the Tragulidse. 



2 The pericardium consists of a parietal and a visceral layer: the former is 

 invested by the mediastinum (see p. 265), and the latter is closely applied to the 

 heart. 



