508 



ANATOMY OF VERTEBRATES. 



334 



when distended, and without a sinus. The pulmonary veins 

 form a common trunk, about half the size of the postcaval, 

 which advances, in contact with and to the left of the post- 

 caval, to open into the hind end of the left auricle, without 

 any valvular structure. The auriculo-ventricular orifice, shaped 

 like that of the right auricle, is close to the termination of 

 the pulmonary vein : it is guarded by a short valve, at the 

 back of the base of the ventricle, to which chordse tendinete are 

 attached. 



The ventricle is conical, with an obtuse apex. More than half 

 the ca^dty, including the apical part, is occupied by a fasciculate 

 decussating muscular structure, from which rises an incomf)lete 

 septum, supporting that between the origins of the pulmonary 

 artery and left aorta. The upper or fore part of the ventricular 

 cavity is formed by a flat sort of platform or roof, supporting the 

 auricular septum, and having the curved auriculo-ventricular slits 

 and valves on each side, with the concavities opposite each other, 

 giving the roof a circular shape. 



The incomplete septum divides the anterior or sternal (pulmonic) 



cavity, whence the pulmonary artery 

 arises, from that at the posterior or 

 dorsal part leading to the orifice of 

 the left aorta, and receiving the blood 

 from the auricles : this ventricular 

 cavity does not extend so near to the 

 apex as the pulmonic cavity does. 

 Tlie part of the ventricle whence the 

 right aorta rises is a still smaller 

 space. A jjair of semilunar valves 

 guards the origin of each artery. 



The heart in Lacertilia essentially 

 resembles that in Opludia, but is 

 shorter in proportion to its breadth. 

 The right auricle, fig. 334, h', is 

 divided by a bivalved orifice from the 

 sinus ; the left auricle, ib. h, has no sinus : it is smaller than the 

 right. Each auriculo-ventricular orifice or slit is guarded by a single 

 valve. The pulmonic cavity of the veutricle, fig. 334, ri, p, is 

 divided from the aortic cavity, ii', by a partial septum, indicated 

 by the dotted outline. The cavity li' receives, as in Opludia, tlie 

 blood from the auricles, and gives oft' the left aorta, a, the right 

 aorta, a', rising from its back part. 



In Chelonia the heart, following, as in Opludia, tlie general 



Heart of Laceria occllata. COLXVII. 



