THE ORGANS. 51 



§ 44. 



Certain portions of the hollow cavitary system, which forms the 

 luenial passages, are converted into contractile vessels by the 

 development of muscles in their walls. The earliest circulatory 

 system arises by these producing by rhythmical action the regular 

 in-and-out flow of the blood. But the direction of the stream of 

 blood is not yet constant ; it can be driven first to one side and then 

 to the other. The portions of the vascular system which are dis- 

 tinguished by their greater contractility are sometimes extended 

 over a large surface, and sometimes limited to shorter parts. They 

 are the beginning of the formation of a heart. 



The heart is therefore an organ differentiated from the blood- 

 vascular passages, and in its simplest form is a portion of the vessels 

 which is able to move its contents in two directions. It is only 

 when valves appear at the ostia of the cardiac tube, that the 

 direction of the flow is defined ; the structure of the heart thus be- 

 comes complicated, and is further elaborated by being divided 

 internally into separate portions (ventricles and am-icles). Contractile 

 organs of this kind often appear as the only differentiated parts of 

 the blood- vascular system, formed from the cavity of the ccelorn. The 

 blood passes directly from the heart into lacunar portions of the 

 coeloin, between the different organs, and from thence back again 

 to the heart (Arthropoda), or there are definite vessels going off 

 from the heart, which sometimes traverse the body in the place of 

 the hollow cavity, or only partially replace the lacunar passage, 

 in that they do not on their way back to the heart reach it as 

 vessels, but into lacunar spaces. In this case the cavity of 

 the coeloni shows itself as a portion of the blood passage, which is 

 only partly represented by true vessels (Mollusca). Where the 

 vessels are completely developed and the heart differentiated, the 

 vascular system is divided into three parts. That which leads from 

 the heart and distributes the blood in the body is called the arterial, 

 and its vessels arteries. The passage which takes the blood back 

 to the centre of the circulation is formed by the veins, and the 

 part of the passage which lies between the afferent and efferent vessels, 

 forms a network of extremely fine canaliculi (capillaries). This 

 intermediate portion is very frequently replaced by a lacunar system, 

 in which case the greater number of the venous passages also have 

 no special walls. 



It is very often difficult to say what should be regarded as a 

 vessel and what as a lacuna, and the distinction often depends on 

 very unimportant points. It is not sufficient to say that the essen- 

 tial character of a vessel is the investment of a cavity by flattened 

 elements derived from modified connective tissue, for these elements 

 might just as fairly be regarded as the covering of the other organs 

 which wall in these spaces ; it is therefore questionable to call wide 

 internal cavities, invested by such cells, "vessels." This cannot 



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