Changes in the blood are not usually considered as organic or structural in character; but, as we have previously seen, the blood is really a fluid tissue, and changes in it embody more or less modification of the character of its constituent elements, as do changes in solid parts. Hence, it appears to us to be perfectly proper to class under this head morbid conditions of this sort. Diseased conditions of the blood are produced in a variety of ways. Perhaps the most frequent means by which the blood becomes diseased is by a retention of the waste products, or excrementitious elements, of the system, which are naturally eliminated as rapidly as produced. The nature of these various elements we have already elsewhere explained and so need only remark that the most important are the following: Uric acid, or urea, a poisonous element eliminated by the kidneys; cholesterine, and other poisonous elements of the bile, eliminated by the liver; carbonic add, eliminated by the lungs; and a variety of poisonous elements eliminated by the skin and by the mucous membrane of the alimentary canal. When the function of any one of these great outlets of the system is suspended, the poisonous elements which it is designed to remove accumulate in the vital fluid, and occasion symptoms of poisoning to a greater or lesser degree. This morbid condition is present in a large share of all general diseases, and is, indeed, one of the most common predisposing causes of disease. The blood may also become diseased by the absorption of poisons from without, as by the reception of poisonous gases, disease germs, and poisonous substances in solution in drinking-water, or taken in conjunction with the food. It is also through the blood that the morbid elements of contagious diseases penetrate the system.

Another mode by which the blood becomes diseased is by a change in the proportion of its constituent elements, by which it becomes unable to perform its functions properly. These changes may consist in an increase or decrease in the proportion of fibrine, of albumen, of water, of salts, of the white globules, or of the red corpuscles. Each of the changes indicated is attended by its particular class of symptoms. When fibrine becomes too abundant, the blood is likely to coagulate in the vessels, forming clots. When it is deficient, the fluidity of the blood becomes so great that severe hemorrhage may result from a very slight wound, or the blood may even ooze through the thin coverings in certain parts of the body, particularly the mucous membrane of the lungs. Deficiency of albumen renders the blood inefficient to support the nutritive processes of the body. When it is too abundant in consequence of overfeeding, the blood becomes too highly charged with nutritive elements, producing feverishness, and even inflammation. This is known as plethora, the opposite of which is anemia. When the fluid portion of the blood is too abundant, as it may become from drinking excessive quantities of fluids, injury may be occasioned by the excessive fullness of the blood-vessels. In the opposite condition the blood becomes thick, and is circulated with difficulty. A deficiency in the number of red corpuscles, a condition usual in debility and deficient nutrition, is usually accompanied with deficient oxygenation of the blood, a function which is chiefly performed by the red corpuscles. This condition is one of the characteristics of anemia. An excessive proportion of white blood corpuscles is also attended by serious interference with the vital functions.

In consequence of these changes in the blood, morbid conditions are produced in all the other fluids, whether secretions or excretions. If the blood contains retained or absorbed poisons, every fluid secretion and excretion will be contaminated with the same. If it is deficient in nutritive elements, the various vital fluids essential to the maintenance of life will also be deficient in the particular elements by which they are characterized, which are derived from the blood. And not only the fluids, but also the solids of the body, must be affected by changes in the blood, since the solids are all produced from the blood.