26 



General Part. 



in the Vertebrata, where they are far more numerous than 

 the white ones. They are found also in many of the lower animals 

 {e.g., certain Ohaetopods and Molluscs), but in these are less 

 regular in form. The plasma and corpuscles together constitute 

 the blood, the colour of which is usually dependent upon the colour 

 of the fluid portion, but in the Vertebrata upon that of the 

 corpuscles. 



The blood corpuscles usually originate in ceUulitr connective tissue, which 

 may constitute specialised organs, the lymph glands. They are therefore 

 connective tissue cells which have broken away from their point of origin and 

 entered the blood stream. 



Since it is of importance that the blood should be in constant 

 movement, hearts are formed, i.e., the vascular walls become very 

 muscular in certain definite regions and can therefore pulsate, 

 or contract rhythmically, and thus drive the blood along. 

 There may be several hearts in the same animal : usually, however, 

 there is only one : or if there are more, one is characterised by its size 

 and strength, and is known as the heart; it is always in direct 

 connection with some of the largest vessels of the body, forming, as it 

 were, the centre of the whole system. The openings into the heart 

 are frequently guarded by peculiar folds, the valves, whose function 

 it is to regulate the blood stream by permitting a flow in one direction 

 only, blocking up the way if the blood tends to stream in the opposite 

 direction (see Fig. 22, l). Very often the heart, consists of several 

 I 2 independent divisions, which are 



frequently regarded as so many 

 aggregated hearts. The blood then 

 usually flows first into a thin walled 

 chamber, the auricle, and from 

 this passes into a thicker walled, 

 more muscular portion, the ven- 

 tricle, which is, apparently, the more 

 important of the two. Sometimes 

 it happens that several auricles open 

 into the ventricle (many Molluscs). 

 The vessels in which the blood flows 

 towards the heart are termed veins, 

 those in which it flows in the opposite 

 direction, arteries. The finest 

 vessels of all, which form a network 

 uniting the ultimate branches of 

 the arteries and veins, are termed 

 capillaries. They lare often 

 altogether wanting, when they are to some extent replaced by 

 portions of the body cavity, with which the veins and arteries then 

 communicate directly, the blood flowing from the arteries into the 

 sinuses and thence into the veins. 



Fig. 22. 1 Diagram of a simple 

 heart ; 2 of one consisting of an auricle 

 and ventricle, a ventricle, v auricle, 

 k valve. — Orig. 



