(Fig. 311, c) situated in the front of the mouth, are separate from the rest, very sharp and pierced by a small canal, which terminates near their extremity; they are fixed upon very small maxillary bones (Fig. 310), and these bones, supported upon a long pedicle, are very movable; so that when the animal does not intend to use these poison-fangs, it turns them backwards, and conceals them in a fold of its gum, from which it can erect them at any moment. One of these long teeth is seen on each side; and there are, behind each of them, several germs, destined to replace it, if it should break in the act of biting; but the maxillary bones do not bear any other teeth; and there are only seen in the roof of the mouth, two rows of palatine teeth, instead of four ranges, as in the common Snakes. These last animals, like several other Reptiles have the palate furnished with teeth, as well as the jaws.-Some Reptiles are, on the contrary, completely destitute of teeth (Frogs, for example); and among Tortoises, which are likewise destitute of these organs, the jaws are covered with a horny plate, like the beak of Birds, having cutting edges; but there are never fleshy and moveable lips as in Mammalia.

477. As the aliment does not require to remain in the mouth, in order to be ground down there, the veil of the palate would have been in general useless; and in fact, it very seldom exists. In the greater number of these animals, the pharynx is not distinct from the mouth, and frequently there is not even any welldrawn line of separation between the oesophagus and the stomach, which is simple and of varied form. The intestines are short and deprived of the cœcal appendage; the large intestine differs little from the small intestine, and terminates in a cloaca, into which open also the urinary canals and organs of reproduction.-Reptiles have, like the higher animals, lymphatic vessels, destined to absorb the products of digestion, and to pour them into the current of the circulation.

478. Their blood is not rich in solid matter; and the oval corpuscles, which swim about in it, are of a greater size than in any other class. The arrangement of the circulating system varies; but there is always a direct communication between the

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system conveying arterial blood, and that conveying venous blood; so that these two liquids mingle, and the body only receives blood which has been imperfectly arterialised by the act of respiration. The heart is almost always composed of two auricles (Fig. 312), opening into a single ventricle. It follows from this

that the arterial blood coming from the lungs, received into the left auricle, and the venous blood flowing from the different parts of the body into the right auricle, are mixed in this common ventricle. One portion of this mixture returns by the aorta to the different organs, which it is destined to nourish; whilst the other part is carried to the lungs by vessels which arise directly from the common ventricle, or even from the aorta. In Crocodiles, however, the heart is formed (Fig. 313) almost in the same manner as in Birds and Mammalia, and presents a partition which separates the right from the left ventricle; it follows, therefore, that this organ presents two distinct ventricles and two auricles, and that the arterial blood is not mixed there with the venous blood; but a particular arrangement of the arteries effects this mixture at some distance from the heart, and the vessels of all the posterior half of the body only receive imperfectly-arterialised blood. In fact, the venous blood received into the right ventricle does not go entirely to the lungs, as in warm-blooded Vertebrata; for, at the side of the opening of the pulmonary arteries (ap), is found another vessel (a) which likewise arises from the right ventricle, and which, after bending back

N N

behind the heart, goes to empty itself into the descending aorta (ao).

ap

od

It follows from this, that at each contraction of the heart, one portion of the venous blood is carried to the lungs, and another portion goes to be mixed with arterial blood; but this mixture is not carried on in the first part of the interior of the aorta, but below the origin of the branches (cc) which this vessel sends to the head and anterior part of the body, so that these parts receive pure arterial blood; whilst all those parts, whose arteries arise below the point of junction of the aorta, with the vessel arising from the right ventricle, only receive a mixture of red and dark blood.It was believed until recently, that among other animals of this class (the Batrachians, or Frog tribe), there only existed a single auricle; but this has been shown to be otherwise. With respect to the mode of distribution of the arteries among Reptiles, we shall only add, that there exist two or more aortic arches curving to the right and to the left, and shortly uniting to constitute one trunk (Fig. 314).

vv, systemic veins, terminating in the right auricle, od; vt, the two ventricles, separated by an internal partition; ap, the two pulmonary arteries, conveying blood from the right ventricle to the lungs; a, trunk from the same ventricle, going to join the descending aorta; vp, pulmonary veins, emptying into left auricle, og; ao, aorta, arising from left ventricle; c, c, vessels proceeding to the head.

479. The respiration of Reptiles is not active; the greater number of these animals consume but little oxygen, and can be deprived of it for a long time without being suffocated. Moreover, temperature has very great influence upon this phenomenon; and in warm weather, the necessity of respiration is felt much more than in winter. A Frog, for example, which has been deprived of air, perishes in summer in less than two hours, whilst in winter it may continue to live for several days. In some Reptiles, as we shall shortly see, gills exist in the early period of life; but the lungs are soon developed, and the gills usually then wither and disappear; so that the same animal has at first

GENERAL CHARACTERS OF REPTILES.

539

an aquatic, afterwards an aerial respiration. There are even

FIG. 314.-CIRCULATING APPARATUS OF LIZARD.

some which retain their organs during the whole of life, and which, having at the same time lungs, are completely amphibious;

the Proteus, Siren, and Axolotl (Fig. 345) are of this number; but most Reptiles have only lungs. It does not follow from

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7, tongue and glottis ; a, esophagus, divided at æ' to show the heart, &e ; i, stomach ; i', intestine; cl, cloaca; an, anus; f, liver; o, ovary; ó, eggs; t, trachea; p, principal lung ; p', undeveloped lung ; vt, ventricle ; c, left auricle ; c', right auricle ; ag, left aortic arch; ad, right aortic arch; d. d, ventral aorta; ac, carotid arteries; E, vena cava superior; vc, vena cava inferior ; vp, pulmonary vein.

this, however, that their respiration is always exclusively aerial; for in several of these animals, the skin is also an organ of respiration, and can act upon the air contained in the water, as well