other important products derived from blood, the removal of effete particles and the substitution of new ones, and all those changes by which the bright blood of the arteries becomes the dark blood of the veins; and again, in the cells of the respiratory organs which the capillaries supply, the dark venous blood is oxygenated and restored to the bright scarlet hue of the arterial blood.

235. Where there are blood-vessels in the lowest animals, the blood is kept in motion by the occasional contraction of some of the principal vessels, as in the worms. Insects have a large vessel running along the back, furnished with valves, so arranged that, when the vessel contracts, the blood can

Fig. 84.

flow only towards the head, and, being thence distributed to the body, is returned again into the dorsal vessel, (Fig. 84,) by fissures at its sides.

236. In all the higher animals there is a central organ, the heart, which forces the blood through the arteries towards the periphery, and receives it again on its return. The HEART is a hollow, muscular organ, of a conical form, which dilates and contracts at regular intervals, independ ently of the will. It is either a single cavity, or is divided by walls into two, three, or four compartments, as seen in the following diagrams. These modifications are important in their connection with the respiratory organs, and indicate the higher or lower rank of an animal, as determined by the quality of the blood distributed in those organs.

237. In the mammals and birds the heart is divided by a vertical partition into two cavities, each of which is again. divided into two compartments, one above the other, as seen in the diagram, Fig. 85.) The two upper cavities are called

auricles, and the two lower ventricles. Reptiles have two

238. The auricles do not communicate with each other, in adult animals, nor do the ventricles. The former receive the blood from the body and the respiratory organs, through veins, and each auricle sends it into the ventricle beneath, through an opening guarded by a valve, to prevent its reflux; while the ventricles, by their contractions, force the blood through arteries into the lungs, and through the body generally.

239. The two auricles dilate at the same instant, and also contract simultaneously; so also do the ventricles. These successive contractions and dilatations constitute the pulsations of the heart. The contraction is called systole, and the dilatation is called diastole. Each pulsation consists of two movements, the diastole or dilatation of the ventricles, during which the auricles contract, and the systole or contraction of the ventricles, while the auricles dilate. The frequency of the pulse varies in different animals, and even in the same animal, according to its age, sex, and the degree of health. In adult man, they are commonly about seventy beats per minute.

240. The course of the blood in those animals which have four cavities to the heart is as follows, beginning with the left ventricle, (Fig. 85, 1. v.) By the contraction of this

ventricle, the blood is driven through the main arterial trunk, called the aorta, (Fig. 90, a,) and is distributed by its branches throughout the body; it is then collected by the veins, carried back to the heart, and poured into the right auricle, (Fig. 85, r a,) which sends it into the right ventricle. rv.) The right ventricle propels it through another set of arteries, the pulmonary arteries, (Fig. 90, p,) to the lungs, (7;) it is there collected by the pulmonary veins, and conveyed to the left auricle, (Fig. 85, 7 a,) by which it is returned to the left ventricle, thus completing the circuit.

241. Hence the blood in performing its whole circuit passes twice through the heart. The first part of this circuit, the passage of the blood through the body, is called the great circulation; and the second part, the passage of the blood through the lungs, is the lesser or pulmonary circulation: this double circuit is said to be a complete circulation. In this case the heart may be justly regarded as two hearts conjoined, and in fact the whole of the lesser circulation intervenes in the passage of the blood from one side of the heart to the other; except that during the embryonic period there is an opening between the two auricles, which closes as soon as respiration commences.

242. In reptiles, (Fig. 86,) the venous blood from the body is received into one auricle, and the oxygenated blood from the lungs into the other. These throw their contents into the single ventricle below, which propels the mixture in part to the body, and in part to the lungs; but as only the smaller portion of the whole quantity is sent to the lungs in a single circuit, the circulation is said to be incomplete. In the Crocodiles, the ventricle has a partition which keeps separate the two kinds of blood received from the auricles; but the mixture soon takes place by means of a special artery, which passes from the pulmonary artery to the aorta.

243. In fishes, (Fig. 87,) the blood is carried directly

from the ventricle to the gills, which are their chief respiratory organs; thence it passes into arteries for distribution to the system in general, and returns by the veins to the auricle. Here the blood, in its circuit, passes but once through the heart; but the heart of a fish corresponds nevertheless to the heart of a mammal, and not to one half of it, as has often been maintained, for the gills are not lungs. 244. Crabs and other crustacea have but a single ventricle, without an auricle.

In the mollusks, there is likewise but a single ventricle, as in Natica, (Fig. 88, h.) Some have in addition one or two auricles. These auricles are sometimes so disjoined

Fig. 88.

as to form so many isolated hearts, as in the cuttle-fish. Among Radiata, the sea-urchins are provided with a tubular heart.

CHAPTER EIGHTH.

OF RESPIRATION.

245. For the maintenance of its vital properties, the blood must be submitted to the influence of the air. This is true of all animals, whether they live in the atmosphere or in the water. No animal can survive for any considerable period of time without air; and the higher animals almost instantly die when deprived of it. It is the office of RESPIRATION to bring the blood into communication with the air.

246. Among animals which breathe in the open air,

S

some have a series of tubes branching through the interior of the body, called trachea, (Fig. 89, t,) opening externally upon the sides of the body, by small apertures, called stigmata, (s;) as in insects and in some spiders. But the most common mode of respiration is by means of LUNGS, a pair of peculiar spongy or cellular organs, in the form of large pouches, which are the more complicated in proportion to the quantity of air to be consumed.

247. In the lower vertebrata, provided with lungs, they form a single organ; but in the higher classes they are in pairs, placed in the cavity formed by the ribs, one on each side of