CIRCULATION. 135 



much darker, being of the shade called by painters Modena red. 

 When first drawn from the vessel, it is a somewhat glutinous and 

 apparently homogeneous fluid, but, after standing for a short time, 

 it separates into two parts, one a watery part, called the serum, the 

 other a more solid part, called the clot, or crassamentum. The 

 serum is chiefly composed of water, with a considerable quantity 

 of the same substance as the white of the egg (albumen) dissolved 

 in it ; so that, if it is exposed to a boiling heat, this coagulates and 

 makes the whole solid. The clot, again, likewise consists of two 

 principal substances, one of which gives it the red color, and, by 

 repeated washings, can easily be separated from the other, which is 

 a white, tough, fibrous matter. It is known by the name of fibrin, 

 and is said to be nearly identical in composition with the part that 

 gives contractility to the muscles.* 



Although the blood in all animals appears to be of essentially 

 the same nature, separating when out of their bodies into a solid 

 and a serous part, yet, in a large proportion of the lower classes, 

 it has not the same florid appearance Which it assumes in most of 

 the Vertebrata. Thus, in insects, this fluid is nearly transparent, 

 while in the caterpillar it has a greenish hue. In fishes, again, it is 

 transparent in the bulk of the body, but it has a red color in the 

 gills, heart, and liver ; and even in the human body, some textures, 

 as the transparent parts of the eye, circulate only a colorless fluid. 

 In certain diseased states of the system, indeed, nearly the whole 

 blood becomes colorless. 



When the blood is examined with a microscope, its florid color is 

 perceived to arise from the numberless extremely minute red 

 globules suspended in the watery serum. These have, in every 

 species where they exist, a determinate size and form, being in 

 man of a circular flattened shape, and from the three-thousandth to 

 the five-thousandth part of an inch in diameter. In birds, reptiles, 

 and fishes, they get progressively larger, assuming at the same time 

 an elliptical form. Their number corresponds very constantly to 

 the temperature of the animal, and hence the two divisions of warm 

 and cold-blooded. In birds the red globules constitute, in general, 



* The following, made by M. Le Canu, is the most recent analysis of the composi- 

 tion of the human blood : Water, 786.500 ; albumen, 69.415; fibrin, 3.555; coloring 

 matter, 119.626; crystallizable fatty matter, 4.300 ; oily matter, 2.270; extractive mat- 

 ter, soluble in alcohol and water, 1.920 ; albumen combined with soda, 2.010 ; chloruret 

 of sodium and potassium, alkaline phosphate, sulphate and subcarbonate, 7.304; subcar- 

 bonate of lime and magnesia, phosphates of lime, magnesia and iron, peroxide of iron, 

 1.414; loss, 2.586. Total, 1000. 



