200 



THE VITAL FUNCTIONS. 



(f,) into the lungs (at h,) where it is aerated, and whence it 

 is reconveyed by the pulmonary veins (i,) into the left au- 

 ricle (k,) which immediately pours it into the left ventri- 

 cle (l,) the point from whence we had set out. 



Both the right and the left heart have their respective au- 

 ricles and ventricles; but they are all united in one envelope, 

 so as to compose, in appearance, but a single organ:* still, 

 however, tlic riglit and left cavities are kept perfectly dis- 

 tinct from one another, and are separated by thick partitions, 

 allowing of no direct transmission of fluid from the one side 

 to the other. These two hearts may, therefore, be com- 

 pared to two sets of chambers under the same roof, having, 

 each, their respective entrances and exits, with a party-wall 

 of separation between them. This junction of the two hearts 



central organ of the nervous system, requires, more than any other part, a 

 supply of oxyg-enated blood for the due performance of its functions. The 

 cui-ious provision which is made for sending this partial supply of blood, of a 

 particular quality, in the larger kinds of reptiles, such as the Crocodile, has 

 been pointed out by many anatomists; but has been lately investigated more 

 particularly by M. Martin St. Ange. (See the Report of G. St. Hilaire, Revue 

 Medicale, for April, 1833.) It is found that in Uicse animals, as well as in 

 the Chelonia, a partial respiratory system is provided for by the admission, 

 through two canals opening externally, of aerated water into the cavity of the 

 abdomen, where it may act upon the blood which is circulating in the vessels. 

 Traces of canals, of this description, are also met with in some of the higher 

 classes of vertebratcd animals, as, for instance, among the MammaUa, in the 

 Monotremaia and the Marsupialia. 



* A remai'kable exception to this 

 general law of consolidation occurs 

 in the heart of the Bugong, repre- 

 sented in Fig. 360, in which it may 

 be seen that the two ventricles, e and 

 L, are almost entirely detached from 

 each other. In tliis figure, which is 

 taken from the Philosopliical Trans- 

 actions for 1820, T) is the systemic 

 auricle, k the riglit or pulmonary 

 ventricle, f the pulmonary arteiy, k 

 the left or pulmonary auricle, l the 

 left or systemic ventricle, and a the 

 aorta. 



