Posterior view of the situation of the more important internal organs: 9. kidnies, right and left; 10. the course of the spinal cord.] [Illustration: Fig. CX. Lateral view of the situation of the more important internal organs.] CHAPTER VI. OF THE BLOOD. Physical characters of the blood: colour, fluidity, specific gravity, temperature: quantity—Process of coagulation—Constituents of the blood: proportions—Constituents of the body contained in the blood—Vital properties of the blood—Practical applications. 211. Supposing the human body to have been built up in the manner now described, and to be in the full exercise of all its functions, the integrity of its various structures is maintained, and their due action excited by the blood. Out of this substance is formed the blandest fluid, as the milk, and the firmest solid, as the compact bone. The heart, capable of untiring action, as long as the blood is in contact with its internal surface, becomes immovable soon after the supply of this fluid is withdrawn; and in less than one minute from the time it ceases to flow in due quantity and of proper quality through the vessels of the brain, the eye is no longer capable of seeing, nor the ear of hearing, nor the brain of carrying on any intellectual operation. 212. At the moment, and for some time after it has issued from its vessel, the appearance of the blood is that of a thick, viscid, and tenacious fluid; yet it is essentially a solid, composed of several substances, each possessing its own distinct and peculiar properties, the relation and combination of which cannot be considered without exciting the feeling that our admiration of the structure of the animal frame ought not to be confined to the mechanism of its solid parts, but that the whole is admirable, from the common material of which it is composed, to its most delicate and elaborate instrument. 213. The colour of redness is universally associated with the idea of blood; but redness of colour is not essential to blood. There are many animals with true, yet without red, blood; and there is no animal in which the blood is red in all the parts of its body. The blood of the insect is transparent; that of the reptile is of a yellowish colour; that of the fish, in the greater part of its body, is colourless. Even the red blood of the human body is not equally red in every part of it, there being two distinct systems of blood-vessels, distinguished from each other by carrying blood of different colours. 214. In the state of health, the specific gravity of human blood, water being 1000, is 1080; from which standard it is capable of varying from 1120, the maximum, to 1026, the minimum. 215. The natural temperature of the human blood is 98°. From this it is capable of varying from 104°, the maximum, to 86°, the minimum; these changes being always the effect of disease.