USES OF THE BLOOD. 689 



through an equal weight of the body in a given time, is proportional 

 to the frequency of the heart's beats. Thus, in 60 seconds, the quan- 

 tities of blood which pass through 1000 parts in weight of the body, 

 in the horse, in Man, in the dog, rabbit, and squirrel, are, respectively, 

 152, 207, 272, 620, and 892 parts, the frequency of the pulse in them 

 being 55, .72, 96, 220, and 320. The ratios, accordingly, are all 

 about as 3 to 1. The more rapid the heart's action, therefore, the 

 quicker must be the nutritive changes in the* tissues of the body; 

 moreover, as there is no evidence of the capillaries being relatively 

 more numerous in the smaller and quick-pulsed animals, the circula- 

 tion through their vessels must be relatively quicker. 



The Uses of the Blood and of its Circulation. 



The blood itself is a highly complex fluid, renewed from, though not 

 altogether formed out of, the lymph and chyle, and perfected, as we 

 shall hereafter see, by the aid of the vascular glands and the respira- 

 tory organs. One of its offices is evidently that of providing an ex- 

 ceedingly elaborate material for the nutrition of every part of the 

 body, arid for the production of the various secretions which are used 

 in the organism. The fluid character of the blood fits it for transmis- 

 sion through a vascular or circulatory apparatus, even through the 

 finest vessels ; and in this circulation through the body, by means of 

 such an apparatus, its proper nutrient materials are conveyed to all 

 the organs and tissues of the frame, into which a fluid plasma passes 

 through the coats of the capillaries. But, secondly, besides furnishing 

 a constant stream of nutrient material, the blood receives, by absorp- 

 tion into its own current, refuse and effete matters from the whole 

 system, and subsequently transports them to proper excreting organs, 

 by which they are thrown out of the body. Lastly, the dark venous 

 blood receives, through the respiratory organs, a quantity of oxygen 

 from the air inhaled into the lungs, and transports this oxygen in the 

 red and arterial blood, to every part of the frame, either for its special 

 stimulation, or for combination with its proximate chemical constitu- 

 ents, in the exercise of the functions of those parts ; the returning 

 venous blood brings back, amongst other oxidized materials, carbonic 

 acid, and conveys this to its proper excreting apparatus, the lungs, 

 whence it is thrown off. The circulating organs in animals are, there- 

 fore, modified, in accordance with the characters of their respiratory 

 organs. 



ORGANS AND FUNCTION OF CIRCULATION IN ANIMALS. 



Vertebrata. The Yertebrata generally, like Man, have not only a perfectly 

 formed blood, containing colored and colorless corpuscles, but they also posses* 

 a central heart placed in a pericardium, and connected with a completely closed 

 system of bloodvessels, consisting of arteries, capillaries, and veins. In the 

 Amphioxus alone, there are no colored corpuscles, and the heart has no peri- 

 cardium. As we have seen, the Vertebrata only have a system of absorbent 

 vessels, which empty themselves into the bloodvessels. The size of the red 

 corpuscles in several animals is given in a Table at pp. 69, 70. 



But important peculiarities exist in the vascular systems of some of the 



44 



