XIII 



PHYLUM CHORDATA 



333 



number of diverticula or air-sacs which are capable of being 

 inflated, causing an increase in the bulk of the animal which 

 doubtless has an effect on assailants. In the snake-like Lizards 

 the right lung is larger than the left, and in the Amphis- 

 ba?nians the latter is entirely aborted. In the Snakes a similar 

 reduction or abortion of the left lung is observable. In the 

 Crocodilia and Chelonia the lungs are of a more complex character, 

 being divided internal!} 7 by septa 

 into a number of chambers. 



Organs of Circulation. --In the 

 heart (Fig. 944) the sinus venosus 

 is always distinct, and is divided 

 into two parts by a septum ; its 

 aperture of communication with the 

 right auricle is guarded by valves. 

 There are, as in the Amphibia, al- 

 ways two quite distinct auricles, the 

 right receiving the venous blood 

 from the body, the left the oxy- 

 genated blood brought from the 

 lungs by the pulmonary veins. But 

 a vital point of difference between 

 the heart of the Reptile and that 

 of the Amphibian is that in the 

 former the ventricle is always more 



/ 



or less completely divided into right 

 and left portions. In all the Lacer- 

 tilia, Ophidia and Chelonia (Fig. 945) 

 the structure is essentially what has 



i/ 



been described in Lacerta, the ven- 

 tricular septum being well-developed, 

 but not completely closing off the 

 left-hand portion of the cavity of 

 the ventricle from the right (cavum 

 pulmonale}. The left-hand portion, 

 which is much the larger, is further 

 imperfectly divided into two parts- 

 the cavum arteriosum on the left 

 and the cavum venosum on the right 



by the two elongated flaps of the auriculo-ventricular valve, 

 which project freely into the cavity of the ventricle. From the 

 cavum pulmonale arises the pulmonary artery, and from the cavum 

 venosum, the two aortic arches. When the auricles contract the 

 cavum venosum becomes filled with venous blood from the right 

 auricle, and the cavum arteriosum with arterial blood from the left- 

 auricle ; the cavum pulmonale becomes filled with venous blood 

 which flows into it past the edges of the incomplete septum. When 



FIG. 944. Heart of Monitor ( Vnranus) 

 dissected to show the cavity of the 

 ventricle and the vessels leading out 

 from it. A. A', auricles; Ao, dorsal 

 aorta ; Ap, Ap f , pulmonary arteries ; 

 Asc. subclavian artery ; Ca. Co! . caro- 

 tids ; RA , RA , roots of dorsal aorta ; 

 Trca, innominate trunk; V, ventricle ; 

 t, right aortic arch ; *, left aortic- 

 arch. (From Wiedersheim.) 



