(1950). Any one who watches a Frog or a Tortoise with a little attention will at once understand the mechanism by which this is effected. The mouth is kept closely shut; and the nostrils, which open immediately into its cavity, are each provided with a muscular valve, so disposed as freely to permit the entrance of air into the mouth, but also effectually preventing its return by the same channel. By this arrangement the descent of the hyoid apparatus fills the mouth with air; and the subsequent contraction of the broad muscles of the throat, the nostrils and the pharynx being of course both closed, forces the air into the opening of the larynx, and distends the lungs, from which it is again expelled by the pressure of the abdominal muscles.

(1951). The structure of the heart and the course of the circulation in Reptiles afford interesting subjects for investigation. The heart consists of three cavities, namely, a strong and muscular ventricle (fig. 342, a), and two membranous and very capacious auricles, both of which communicate by valvular openings with the ventricular cavity. The right auricle (6) receives the venous blood from all parts of the body through the venae cavae (n, o, p), the terminations of which are guarded by strong valves; the left auricle (c) is appropriated exclusively to the lungs, from which it receives arterial blood through the pulmonary veins (m m.) It is obvious, therefore, that the ventricle receives two kinds of blood from the two auricles - venous blood from the systemic auricle, and arterial blood from the pulmonic auricle; and as the interior of the ventricular cavity is crossed by innumerable columnae carnece, giving it almost a spongoid appearance, the vitiated and purified blood derived from these two sources are more or less completely mixed together, and blood only partially arterialized is distributed to the system.

Heart of the Tortoise.

Fig. 342. Heart of the Tortoise.

(1952). Two sets of vessels take their origin from the single ventricle, viz. the pulmonary and aortic. The pulmonary artery soon divides into two trunks (ff), one destined to each lung; so that a part of the impure blood expelled from the ventricle is at once driven to the organs of respiration to be further oxygenized. The aorta, immediately after its origin, likewise separates into two trunks (d, e), the right and the left, which, winding backwards, ultimately join to form one great vessel (I), from which the arteries of the viscera (i, 7c) and those destined to the posterior parts of the body are given off. From the commencement of the right aortic trunk a very large vessel is furnished, which bifurcates to form two arterial innominatce (g g), from which the carotid and subclavian arteries take their origin.

(1953). Although the above description refers more immediately to the construction of the heart of the Tortoise, in all essential particulars it is equally applicable to all reptiles of the Saurian, Chelonian, and Ophidian orders; and when we thus see that, in addition to the comparatively imperfect condition of their lungs, the blood which circulates through the body is in these creatures a mixed and semivenous fluid, we need not be surprised at the contrast which they offer when compared with the hot-blooded and vigorous animals to be described in the subsequent chapters of this work.

(1954). Cuvier committed a serious error in describing the Batrachian reptiles as having a heart composed of but two cavities; our illustrious countryman John Hunter had already ascertained that, in Frogs, Toads, and Salamanders, the heart possessed a pulmonary as well as a systemic auricle; and his observations have since been abundantly confirmed by Dr. Davy, Dr. Martin St.-Ange, and Professor Owen. The pulmonic auricle in these creatures, indeed, is comparatively of small size; but it exists as a perfectly distinct chamber, and receives the blood from the lungs preparatory to its admission into the common ventricle.

(1955). With regard to the use of the additional auricle in the Rep-tilia, Professor Owen has well remarked* that, from the impediments which frequently occur to a free and regular circulation of blood in these cold-blooded and slow-breathing creatures, the venous side of the heart is subject to great distension; hence the large size of the auricles, and of the sinus which receives the systemic veins, and also the perfect development of the valves intervening between the venae cavaa and the auricle, of which the Eustachian valve of the Mammiferous heart still presents a rudiment. Had the pulmonary veins terminated along with the systemic in the same cavity, their orifices would have been subjected to the pressure of the accumulated contents of that cavity, and there would have been a disproportionate obstacle to the passage of the aerated blood into the ventricle. This is obviated by providing the pulmonary veins with a distinct receptacle, which is equally ready with the right auricle to render its contents into the ventricle during the diastole of that cavity.

* Transactions of the Zoological Society of London, vol. i. p. 217.

(1956). Passing from the consideration of the more perfect Reptile circulation as it exists in those genera which in their adult condition possess lungs only, to those which may properly be called Amphibious, and are provided with both lungs and gills throughout the whole period of their lives, we must still pause to notice one or two intermediate forms, which, notwithstanding that they lose their branchiae at an early stage of their growth, are evidently closely related to the Perenni-branchiata, as may be gathered from the arrangement which their blood-vessels permanently exhibit. Such is the Menopoma, or Great South American Salamander, an animal met with in the rivers and lakes of the South American continent. In the annexed figure, taken from the Catalogue of the Hunterian Collection, the principal vessels of this creature are delineated as seen from the dorsal aspect. The lower jaw (fig. 343, a) has been removed from the head, so that in the drawing are exposed the cut edge of the masseter muscle (6), the tongue (c), and the opening of the larynx, into which a bristle (d) has been introduced, one end of which is seen passing into the cavity of the right lung: the bag of the pharynx (ff) has been left entire; and upon this the main vascular trunks are supported. From the heart, situated upon the opposite side of the oesophagus, is given off a large vessel representing the bulbus arteriosus of fishes, which terminates by dividing into four branchial arteries; but as in the adult Menopoma there are no branchiae, these vessels (o o o) wind round each side of the neck, and again unite into two trunks (r r) which by their nnion form the aorta (t t.) It will easily be perceived that this arrangement is precisely that met with in fishes; only that, as there are here no gills intervening between the terminations of the branchial arteries and the commencements of the branchial veins, these vessels are immediately continuous with each other.