460 THE POPULAR SCIENCE MONTHLY. 



THE BLOOD AXD ITS CIRCULATIOX. 



By IIEEMAN L. FAIRCHILD. 



THE main facts of blood circulation have been known only two 

 hundred and fifty years. This w^ould be surprising if we were 

 not aware that most of our certain knowledge in natural history, in- 

 cluding many truths of easier discovery than the circulation of the 

 blood, has been gained within the last one hundred years. And, in- 

 deed, the blood and its movements are not yet fully understood. Sev- 

 eral points which, at first thought, would seem of easy solution, are 

 matters in dispute or confessed mysteries. The purpose of this article 

 is, not to publish new truth or discuss difficult points, but to compactly 

 present the fundamental and interesting facts relating to the circula- 

 tion in all animals. 



The necessity of a circulating nutritive fluid lies in the localizing 

 of the process of digestion. In proportion as digestion and absorp- 

 tion of food become specialized and restricted to certain parts, circula- 

 tion becomes more important in order to convey that food to the tis- 

 sues, and carry from the tissues the worn-out material. To maintain 

 the character of the fluid, it must itself undergo constant change, and 

 hence the excretory processes respiration being the most urgent 

 which increase the necessity for movement of the fluid. Circulation 

 of the nutritive fluid is the immediate function for upbuilding and re- 

 pairing the body. It harmonizes the several vegetative functions, and 

 should be regarded as the primary function, to which all the others 

 are subservient. 



The amoeba, sponge, and tapeworm have no blood ; they have no 

 necessity for it, as they are destitute of digestive organs, their food 

 being in immediate contact with all parts of the body : or, we might 

 regard their blood as simply the water or fluid in which the animal is 

 immersed. In animals possessing the simplest digestive cavities, as the 

 jelly-fish and sea-anemone, the blood is merely the dissolved food, cor- 

 responding to the cJtyme of higher animals. In the starfish, sea- 

 urchin, and other invertebrates, having a complete and distinct stom- 

 ach, the blood is chyle ; while in vertebrates the blood is a distinct 

 fluid, chemically very complex, difficult of analysis, and not perfectly 

 understood : structurally, it is essentially the same in all animals a 

 clear fluid containing organic particles. 



The blood contains all the nourishment which supports the various 

 tissues of the whole structure. It may properly be regarded as the 

 fundamental tissue, and is well named in the French chaw coulant 

 running flesh. It changes rapidly by eating, exercise, and any influ- 

 ence which affects the supply of nutriment or the waste of the body. 

 It is derived primarily from the new food, received in the higher ani- 



