(2328). Neither will it be at all necessary to describe at any length the construction of the respiratory and circulatory organs in the class now under consideration, seeing that the structure of the lungs, the mechanism of respiration, the arrangement of the pulmonary vessels, the cavities of the heart, and the general disposition of the arteries and veins of the systemic circulation differ in no material circumstance from what is met with in the human subject.

(2329). The lungs, occupying the two sides of the chest, are each contained in a distinct chamber, formed by the ribs and diaphragm, without in any part adhering to its walls. Each lung is enclosed in a serous cavity formed by the pleura, which, after lining the ribs, the intercostal muscles, and the thoracic surface of the diaphragm, is reflected on to the lung itself at the point occupied by the roots of the pulmonic vessels, and invests the entire surface of the viscus; it moreover passes deeply into those fissures that separate the lung into several distinct lobes.

(2330). In the interspace between the two pleurae, called the mediastinal is lodged the heart, contained in a fibro-serous envelope (the pericardium); and behind this, the oesophagus, accompanied by the principal trunks of the vascular system, passes through the thorax into the abdomen.

(2331). Each lung is a closed bag, composed of innumerable cells that communicate with the terminations of the bronchial tubes, and collectively present an immense surface, over which the blood contained in the capillaries of the pulmonary vessels is made to circulate.

(2332). The inspiration and expiration of air are effected by the alternate movements of the diaphragm and of the walls of the thoracic cavity, whereby the atmospheric fluid is drawn into and expelled from the pulmonary cellules, and is thus constantly renewed as it becomes deteriorated by the abstraction of the oxygen consumed during the process of converting the venous into arterial blood.

(2333). The purified blood, after passing through the pulmonary capillaries, is collected in an arterialized condition by the pulmonary veins, and conveyed to the systemic side of the heart, which offers the same arrangement throughout the entire class, consisting of an auricular chamber (fig. 403, c) and of a very muscular ventricle (a), the auriculo-ventricular opening being guarded by mitral valves and columnce carnece, similar to those found in the human heart. From the left ventricle the blood is driven into the aorta (e), the commencement of which is guarded by three semilunar valves, and thus it passes through the entire system.

(2334). When again collected from the periphery of the body, the now vitiated fluid is returned to the heart by the venous system, and poured through the venae cavae into the right or pulmonic auricle; and hence it passes into the right ventricle (fig. 403, b), to be again returned through the pulmonary artery to the lungs, thus completing the circulation.

(2335). But although the general arrangement of the circulatory and respiratory organs in all Mammals thus in every respect resembles that which exists in the human body, there are of necessity variations in the distribution of certain parts of the sanguiferous system, adapted to the peculiarities of organization presented by the different orders and even families of this great class, which must not be wholly passed over in silence.

(2336). In the CETACEA, for instance, many interesting circumstances are observable in the arrangement of the vascular system.

(2337). In the herbivorous genera, as for example in the Dugong, the two sides of the heart are separated to a considerable extent by a deep fissure (fig. 403, a, b), so that the pulmonary and systemic hearts are much more evidently distinct viscera than they appear to be in the quadrupedal forms; nevertheless in the Whalebone and Spermaceti Whales the heart assumes the usual appearance, and is only remarkable for its amazing size; this, indeed, may well have attracted the notice of Hunter* while investigating such gigantic beings. "In our examination of particular parts," says that eminent anatomist, "the size of which is generally regulated by that of the whole animal, if we have only been accustomed to see them in those which are small or middle-sized, we behold them with astonishment in animals so far exceeding the common bulk as the Whale. Thus the heart and aorta of the Spermaceti Whale appeared prodigious, being too large to be contained in a wide tub, the aorta measuring a foot in diameter. When we consider these as applied to the circulation, and figure to ourselves that probably ten or fifteen gallons of blood are thrown out at one stroke, and moved with an immense velocity through a tube of a foot in diameter, the whole idea fills the mind with wonder".

Heart of the Dugong.

Fig. 403. Heart of the Dugong.

* The Animal (Economy, by J. Hunter, with Notes by Professor Owen, p. 366.

(2338). In the arrangement of the blood-vessels of the Cetacea, many interesting peculiarities are met with *. The general structure of the arteries, indeed, resembles that of other Mammals, and, where parts are nearly similar, their distribution is likewise similar. But these animals have a greater proportion of blood than any others known; and there are many arteries apparently intended as reservoirs, wherein a large quantity of arterial blood may accumulate apparently for important purposes, where vascularity could not be the only object. Thus the intercostal arteries divide into a vast number of branches, which run in a serpentine course between the pleura and the ribs, and penetrate the intercostal muscles, everywhere lining the walls of the thorax. These plexiform vessels, moreover, pass in between the ribs near their articulation, and anastomose extensively with each other. The medulla spinalis is likewise surrounded with a network of arteries in the same manner, more especially as it comes out from the brain, where a thick substance is formed by their ramifications and convolutions; and these vessels most probably anastomose with those of the thorax.