toward each other, so that the upper surface of the clot becomes more or less excavated or cup-shaped. (Fig. 66.) The blood is then said to be "buffed and cupped." These appearances do not present themselves under ordinary conditions, but only when the blood has become altered by disease.

The entire quantity of blood existing in the body has never been very accurately ascertained. It is not possible to extract the whole of it by opening the large vessels, since a certain portion will always remain in the cavities of the heart, in the veins, and in the capillaries of the head and abdominal organs. The other methods which have been practised or proposed from time to time are all liable to some practical objection. We have accordingly only been able thus far to ascertain the minimum quantity of blood existing in the body. Weber and Lehmann' ascertained as nearly as possible the quantity of blood in two criminals who suffered death by decapitation; in both of which cases they obtained essentially similar results. The body weighed before decapitation 133 pounds avoirdupois. The blood which escaped from the vessels at the time of decapitation amounted to 12.27 pounds. In order to estimate the quantity of blood which remained in the vessels, the experimenters then injected the arteries of the head and trunk with water, collected the watery fluid as it escaped from the veins, and ascertained how much solid matter it held in solution. This amounted to 477.22 grains, which corresponded to 4.38 pounds of blood. The result of the experiment is therefore as follows:Blood which escaped from the vessels remained in the body

66

12.27 pounds.

4.38 66

Whole quantity of blood in the living body, 16.65

The weight of the blood, then, in proportion to the entire weight of the body, was as 1 : 8; and the body of a healthy man, weighing 140 pounds, will therefore contain on the average at least 171 pounds of blood.

'Physiological Chemistry, vol. i. p. 638.

CHAPTER XII.

RESPIRATION.

THE blood as it circulates in the arterial system has a bright scarlet color; but as it passes through the capillaries it gradually becomes darker, and on its arrival in the veins its color is a deep purple, and in some parts of the body nearly black. There are, therefore, two kinds of blood in the body; arterial blood, which is of a bright color, and venous blood, which is dark. Now it is found that the dark-colored venous blood, which has been contaminated by passing through the capillaries, is unfit for further circulation. It is incapable, in this state, of supplying the organs with their healthy stimulus and nutrition, and has become, on the contrary, deleterious and poisonous. It is accordingly carried back to the heart by the veins, and thence sent to the lungs, where it is reconverted into arterial blood. The process by which the venous blood is thus arterialized and renovated, is known as the process of respiration.

This process takes place very actively in the higher animals, and probably does so to a greater or less extent in all animals without exception. Its essential conditions are that the circulating fluid should be exposed to the influence of atmospheric air, or of an aerated fluid; that is, of a fluid holding atmospheric air or oxygen in solution. The respiratory apparatus consists essentially of a moist and permeable animal membrane, the respiratory membrane, with the blood vessels on one side of it, and the air or aerated fluid on the other. The blood and the air, consequently, do not come in direct contact with each other, but absorption and exhalation take place from one to the other through the thin membrane which lies between.

The special anatomical arrangement of the respiratory apparatus differs in different species of animals. In most of those inhabiting the water, the respiratory organs have the form of gills or branchiæ ; that is, delicate filamentous prolongations of some part of the

integument or mucous membranes, which contain an abundant supply of blood vessels, and which hang out freely into the surrounding water. In many kinds of aquatic lizards, as, for exam

Fig. 67.

HEAD AND GILLS OF MENOBRANCHUS.

ple, in menobranchus (Fig. 67), there are upon each side of the neck three delicate feathery tufts of thread-like prolongations from the mucous membrane of the pharynx, which pass out through fissures in the side of the neck. Each tuft is composed of a principal stem, upon which the filaments are arranged in a

pinnated form, like the plume upon the shaft of a feather. Each filament, when examined by itself, is seen to consist of a thin, ribbon-shaped, double fold of mucous membrane, in the interior of which there is a plentiful network of minute blood vessels. The dark blood, as it comes into the filament from the branchial artery, is exposed to the influence of the water in which the filament is bathed, and as it circulates through the capillary network of the gills is reconverted into arterial blood. It is then carried away by the branchial vein, and passes into the general current of the circulation. The apparatus is further supplied with a cartilaginous framework, and a set of muscles by which the gills are gently waved about in the surrounding water, and constantly brought into contact with fresh portions of the aerated fluid.

Most of the aquatic animals breathe by gills similar in all their essential characters to those described above. In terrestrial and air-breathing animals, however, the respiratory apparatus is situated internally. In them, the air is made to penetrate into the interior of the body, into certain cavities or sacs called the lungs, which are lined with a vascular mucous membrane. In the salamanders, for example, which, though aquatic in their habits, are air-breathing animals, the lungs are two long cylindrical sacs, running nearly the entire length of the body, commencing anteriorly by a communication with the pharynx, and terminating by rounded extremities at the posterior part of the abdomen. These lungs, or air-sacs, have a smooth internal surface; and the blood which circulates through their vessels is arterialized by exposure to the air contained in their cavities. The air is forced into the lungs by a kind of

swallowing movement, and is after a time regurgitated and discharged, in order to make room for a fresh supply.

In the frog, tortoise, serpents, &c., the structure of the lung is a little more complicated, since respiration is more active in these animals, and a more perfect organ is requisite to accomplish the arterialization of the blood. In these animals, the cavity of the lung, instead of being simple, is divided by incomplete partitions into a number of smaller cavities or "cells." The cells all communicate with the central pulmonary cavity; and the partitions, which join each other at various angles, are all composed of thin, projecting, double folds of the lining membrane, with blood vessels ramifying between them. (Fig. 68.) By this arrangement, the extent of surface presented to the air by the pulmonary membrane is much increased, and the arterialization of the blood takes place with a corresponding degree of rapidity.

LUNG

face.

Fig. 68.

OF FROG,

In the human subject, and in all the warmblooded quadrupeds, the lungs are constructed on a plan which is essentially similar to the above, and which differs from it only in the greater extent to which the pulmonary cavity is subdivided, and the surface of the respiratory membrane increased. The respiratory apparatus showing its internal sur(Fig. 69) commences with the larynx, which communicates with the pharynx at the upper part of the neck. Then follows the trachea, a membranous tube with cartilaginous rings; which, upon its entrance into the chest, divides into the right and left bronchus. These again divide successively into secondary and tertiary bronchi; the subdivision continuing, while the bronchial tubes grow smaller and more numerous, and separate constantly from each other. As they diminish in size, the tubes grow more delicate in structure, and the cartilaginous rings and plates disappear from their walls. They are finally reduced, according to Kölliker, to the size of of an inch in diameter; and are 25 composed only of a thin mucous membrane, lined with pavement epithelium, which rests upon an elastic fibrous layer. They are then known as the "ultimate bronchial tubes."

Each ultimate bronchial tube terminates in a division or islet of the pulmonary tissue, about of an inch in diameter, which is termed a "pulmonary lobule." Each pulmonary lobule resembles in its structure the entire frog's lung in miniature. It consists of a

HUMAN LARYNX, TRACHEA, BRONCHI, AND LUNGS; showing the ramification of the bronchi, and the division of the lungs into lobules.

vascular membrane inclosing a cavity; which cavity is divided into a large number of secondary compartments by thin septa or partitions, which project from its internal surface. (Fig. 70.) These

Fig. 70.

a

SINGLE LOBULE OF HUMAN LUNG.-a. Ultimate bron

secondary cavities are the "pulmonary cells," or "vesicles." Each vesicle is about

of an inch in diameter; and is covered on its exterior with a close network of capillary blood vessels, which dip down into the spaces between the adjacent vesicles, and expose in this way a double surface to the air which is contained in their cavities. Between the vesicles, and in the interstices between the lobules, there is a large quantity of yellow elastic tissue, which gives firmness and resiliency to the pulmonary structure. The pulmonary vesicles, according to the observations of Kölliker, are

chial tube b. Cavity of lobule. lined everywhere with a layer of pavement epithelium, continuous with that in the

c, c, c. Pulmonary cells, or vesi

cles