carried into the right auricle. Thus we have it, 1st. from trunks to branches of the artery; 2dly, from the branches to the trunk of a vein; 3dly, from the trunk of that vein, by its branches sent to the liver; and, lastly, taken up by veins, and conveyed to the cava. Our cut will illustrate this, where a is intended for the mesenteric artery, 6 the porta of the abdomen collecting the blood from the viscera, c the porta of the liver distributing it through that viscus, d the cava of the liver conveying it into the ascending cava e. In this way the course of the blood is much lengthened, and from the greater portion being in veins, it must be comparatively sluggish. The first and second set of veins are collectively termed the porta or vena portæ, or the vein of the gate,—a name given by the ancients from its entering under a portion of the liver which they called the gate. That portion which takes up the blood from the extremities of the arteries is termed the porta of the abdomen; the other, which transmits it through the liver, is called the porta of the liver, and the vein by which it is collected from the liver is called the cava of the liver.

The mode of ramification of the arteries of the intestinal canal deserves notice. In the human subject it divides into primary branches, which establish very free communications with each other, forming what are termed arches; from the convexities of these arches other secondary branches are sent off, which in a similar manner form arches, and so on till they reach the intestine, forming generally three or four series of arches before they do so, and constituting the best example we have in the human body of the net-work, which was stated as being calculated to diminish the force and velocity of the blood, and also take off the pulsative movement. Thus the blood will flow to the intestines in a tranquil uniform stream, adapted to the slow continuous motion of the intestine. Moreover, had the blood been propelled rapidly

along these arteries, how could such rapid movement accord with what must necessarily be a very slow, languid, and sluggish stream in the venous system beyond? In carnivorous animals, which have quick digestion, the arches are not so numerous, nor the communications so free; while in herbivorous animals they are more frequent than in the human species. In the pig both arteries and veins present a very remarkable distribution: they are closely and most intricately interwoven, so as to present one of the most remarkable examples of vascular arrangement in the whole animal kingdom. The adjustment of this peculiar abdominal circulation to the condition of the different organs will be further considered when we come to digestion.

Having now brought under our consideration the general structure and mode of action of the different organs engaged in the distribution of the blood through the body; having adverted to certain mechanical conditions to which they are subjected, and the principal modifications they undergo, so as to adapt them to particular organs and conditions, our attention is next to be directed to the function of respiration, which, in one point of view, may be considered as subsidiary to circulation, in so far as one of its most important offices is to prepare the blood while passing through the lesser circulation for its subsequent course through the greater.

44

CHAPTER II.

RESPIRATION.

Organs of Inspiration-Expiration-Passages through the Diaphragm—Incidental Respiration-Respiration in Birds-in Reptiles-Opinions of the Ancients as to the use of Respiration-Atmosphere-Its Height-Weight-HumidityTemperature-Composition-Changes produced on the Air by breathing— Quantity of Water thrown off in the Breath-Diminution of Oxygen-Increase of Carbonic Acid-State of Nitrogen-Changes upon the Blood-In Colour— Density-Chemical Constitution-Temperature and Animal Heat-Source and Distribution of-How regulated when the Body is exposed to High and to Low Temperatures-Progressive Developement of Animal Heat in the New-born→ Adaptation to the Temperature of different Climates-Hybernation-Voice and Speech-Respiration of different Gases-Asphyxia-Means for restoring the Functions in Suspended Animation.

RESPIRATION is that function in living bodies by which the circulating fluids are brought under the influence of the atmospheric air, during which the fluid exposed to the air, and the air itself, mutually act on each other. The organs engaged in this function present great diversities, according to the rank the individual holds in the organic kingdom. In man, mammalia, and birds, respiration is performed by the lungs, and the alternate enlargement and diminution of the cavity in which they are contained. In reptiles the air is forced down, not drawn into the lungs. In fishes the gills perform this office. Animals still lower in the scale present various modes in which it is performed; and in plants the leaves are its principal agents. We shall chiefly confine ourselves to the consideration of this function as it obtains in man, though

for the purpose of illustration other animals will be occasionally referred to.

The organs engaged in respiration are the direct and the accessary; the lungs being the former, the organs by which the air is introduced into the lungs constituting the latter. The chest or thorax is of the form of a cone, with the apex cut off, approaching in the human subject, and particularly in the male, to a quadrangular pyramid. The skeleton of its walls is composed of twelve vertebræ, twelve ribs on each side, with their cartilages, and the breast-bone in front. It is separated from the cavity of the abdomen by a muscular partition called the diaphragm. The vertebræ are termed dorsal, to distinguish them from seven of the neck and five of the loins, which together form the flexible spine. These dorsal vertebræ are so united or hinged together as to permit of a less degree of motion than those of either the neck or loins. The ribs are of an arched form, the first forming the segment of the smallest circle, the circle gradually enlarging downwards. Their upper margin is rounded, and has an inclination inwards; the lower is sharp, and inclines outwards. At their attachment behind to the vertebræ they incline from above downwards and forwards. From these circumstances, when they are raised, the chest is enlarged. Excepting the two last, they have two points of connection with the vertebræ behind. When they come forwards, they terminate in cartilages or gristle, by which they are connected to the breast-bone or to each other, excepting the last two, which float loosely. Anteriorly, from the first downwards, the cartilages become gradually longer, more slender, and flexible. From the con

nection behind, and from the termination in front, the upper rib is always more fixed than that immediately beneath it. These points, along with the segment of the circle which they form enlarging downwards, we shall find have a considerable influence on their office.

There are two conditions of respiration which are ne

cessary to be considered-ordinary and incidental. Ordinary respiration commences at birth, and must be carried on to the last; it is conducted whether we are asleep or awake. In so far, therefore, it is like the function of circulation, an involuntary act. But we possess a direct power over breathing which we have not over circulation. To a certain extent we can hurry or retard, or suspend it for some time, either when the chest is expanded or contracted, or expel the air with different degrees of force. This extraordinary or incidental respiration is employed in speaking, singing, laughing, crying, coughing, straining, &c. In both conditions respiration consists of two acts: inspiration, during which the chest is enlarged, and an additional portion of air drawn in; and expiration, when the chest is contracted and a portion of air expelled.

The chest is enlarged by the elevation of the ribs and descent of the diaphragm. By the former it is enlarged from before backwards, and from right to left; and by the latter from above downwards, so that it is thus enlarged in all its diameters. When the ribs are elevated, their points of connection behind are not so, and the seven superior, from being fixed anteriorly to the breast-bone, are elevated at neither extremity. From the form and position of the ribs, it is by the change of the plane of their inclination, when they are depressed or elevated, that they produce a difference in the capacity of the chest. In ordinary inspiration a set of muscles termed intercostals are the only ones required for the elevation of the ribs. These muscles present us with a very beautiful example of design and contrivance. They consist of two layers, lying one upon the other, their fibres running in different directions. The external layer begins behind at the vertebræ, and runs downwards and forwards from the inferior margin of the upper to the superior margin of the lower rib. This layer is not continued the whole length of the ribs, but ceases