36 INTRODUCTION. 



quids from every part of the body to this same centre, are 

 named veins; at the point of reunion of both, is to be found in 

 many animals a fleshy organ, the heart, which aids by its con- 

 tractions the motion of the liquid, and which, like the vessels, 

 is more or less complicated. We find the first rudiments of 

 vessels in some intestinal worms, and the first rudiment of a 

 heart in insects. 



In the annelides, the only invertebrated animals which have 

 red blood, there are arteries and veins for the circulation, but 

 there is simply a rudiment of a heart. In the arachnides tra- 

 cheariae, the organs of circulation are not any better marked 

 than in the insects; but in others, such as the pulmonariae, there 

 is a heart or great dorsal vessel and branches on each side. 



The Crustacea present more distinctly a heart; in some it is 

 elongated into a large fibrous vessel which extends all along 

 the tail, giving branches on both sides, and which recall to 

 our minds the dorsal vessel of insects; but in other Crustacea, 

 there is a dorsal ventricle, a great abdominal vessel, and posi- 

 tive circulatory vessels. In the mollusca there is a heart more 

 or less complicated, a double system of arteries and veins; the 

 blood is white or bluish. Finally in the vertebrata, besides 

 the arteries, veins and heart, there is a particular system of 

 lymphatic and chyliferous vessels which convey the nourish- 

 ing fluid from the intestines into the veins. 



The simplest heart is composed at least of a ventricle which 

 propels the blood into the arteries, and is often accompanied 

 with an auricle or venous sinus at their entrance into the heart; 

 it is called aortic when it sends the blood to the whole body, 

 and pulmonary when it sends it to the respiratory organs; it 

 is double when there are two ventricles, which, however, may 

 be separated or united. The heart is simple without auricle 

 and pulmonary, in all the articulated animals which are pro- 

 vided with one. The same is the case in fishes, with the ex- 

 ception of there being an auricle. The heart is simple but 

 aortic in most mollusca; it is triple in the cephalopodous mol- 

 lusca, in which there are two pulmonary ventricles and one 

 aortic, separated and without auricles. In all reptiles there is 

 one ventricle only, more or less divided by a partition, and 



