CHAPTER VI 



TRANSPORT OF MATERIALS THE VASCULAR 



SYSTEM 



IN unicellular animals and in the more primitive small multi- 

 cellular animals there is no need for the provision of special means 

 of conveying chemical products from one part to another, since 

 they readily pass by diffusion. But when, for example, the 

 materials derived from the digestion of food are prepared in one 

 particular part of the organism at a distance from other parts 

 requiring them, special channels and means of transport are needed, 

 just as we saw was the case with oxygen. And, as in that case, 

 the means of transport is the blood. We have now to inquire 

 how this transport is effected. It is clear that the blood must be 

 sent in a current, so that its constituents may reach all organs, and 

 that the same blood must circulate since there is no loss of it. 



At a very early stage of evolution we find a muscular tube 

 which, by rhythmical contractions, causes currents of a more or less 

 irregular nature in the liquid of the body cavity. This tube is 

 open at both ends, but may be regarded as a rudimentary kind of 

 heart, although the fluid which it drives is not confined to any 

 particular channels, such as we find in the blood vessels of the 

 more highly organised animals. In its most perfect form, as in the 

 mammals, the general arrangement may be represented as in the 

 diagram of Fig. 7. In this figure, for the sake of simplicity, the 

 hollow muscular organ, known as the heart, is represented as two 

 separate organs, left and right. Although the two parts are united 

 in one mass, their cavities are quite distinct and separate. Starting, 

 then, from the left side of the heart, at the upper right-hand corner 

 of the diagram, we note that blood, which has replenished its 

 oxygen and got rid of a large part of its carbon dioxide in the 

 lungs, enters the contractile cavity, known as the left ventricle. But 

 immediately before it enters the ventricle it passes through another 

 chamber, the auricle, with thinner walls, but also contracting 

 rhythmically, immediately before the ventricle. By this means 

 the ventricle is filled up with blood. This ventricle then contracts 

 with force, and as there are valves between it and the auricle which 



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