STBTJCTUKE OF THE HEAET IN EEPTILES. 697 



of the ventricular cavity is crossed by innumerable columnce 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. 



(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 (//), one destined to each lung ; so that a part of the im- 

 pure 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 

 (1), 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 bifur- 

 cates to form two arterice innominatce (y 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 com- 

 paratively 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 sub- 

 sequent 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 de- 

 velopment of the valves intervening between the venae cavae 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 

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



