H. ORGANS OF CIRCULATION. 



(VASCULAR SYSTEM.) 



THE organs of circulation, which arise from the mesoblast, 1 

 consist, in the Craniata, of a hollow central muscular organ, the 

 heart, which is connected with a series of completely closed tubes, 

 the blood-vessels. The heart and blood-vessels contain a coloured 

 fluid, the Uood, and their cavities probably represent the remains 

 of the blastoccele (p. 4). Another system of vessels containing a 

 colourless fluid, the lymph, must be distinguished from the blood 

 vessels : lymph, however, is present in various spaces or sinuses 

 in the body as well as in the lymph -vessels (p. 333) : the lymphatic 

 system is, .therefore, not completely closed, the vessels communicat- 

 ing with the sinuses on the one hand, and with the blood-vessels on 

 the other. 



Both blood and lymph consist of a colourless fluid, the plasma, 

 in which float numerous cells or corpuscles. The blood-corpuscles are 

 of two kinds colourless, nucleated, amoeboid cells, known as white 

 or colourless corpuscles or leucocytes, and far more numerous red 

 Hood- corpuscles or erythrocytes? The lymph contains colourless 

 corpuscles only, and these are precisely similar to those of the 

 blood. Both blood and lymph are kept in constant circulation 

 through the vessels by the contraction of the heart, which acts 

 both as a force-pump and a suction-pump, and they serve to carry 

 the absorbed food and oxygen to, and the waste products from, all 

 parts of the body. 



All the blood vessels which bring back the blood to the heart 

 are known as veins, while those which carry it from the heart 

 are called arteries : the latter usually contain oxygenated, the 

 former impure blood, but this is by no means always the case. 

 Many of the veins are provided with valves, which are adapted to 

 prevent the reflux of the blood : they have the form of semilunar 

 folds of the internal coat, and each is usually made up of two folds, 



1 According to some embryologists the hypoblast also takes part in the forma- 

 tion of the vascular system. 



2 In Amphioxus the blood contains white corpuscles only ; there is no heart, 

 and the vessels are only partially comparable to those of the Craniata. 



