88 



It is curious, however, that in the whole of this class the descending 

 aorta is formed by the union of two branches, the right branch 

 arises from the left ventricular compartment, consequently carries 

 pure, or nearly pure blood, which it distributes to the head, neck, 

 chest, and upper extremities. The left branch, on the contrary, 

 arises either from the right ventricular compartment, or what is 

 tantamount to it, from the pulmonary artery. It is obvious, then, 

 that the descending aorta carries a mixed quality of blood to the 

 parts it supplies; but it is interesting to observe, that previous to 

 the junction of the two aortic arches, the left gives off the cosliac 

 axis which supplies the entire alimentary canal and digestive organs 

 with venous blood. In the turtle, lizard, and some serpents, where 

 the septum ventriculorum is imperfect, the pulmonary artery and 

 aorta at once carry mixed blood, and in some of the chelonia, 

 as the tortoise, the existence of ductus arteriosus ensures a more 

 complete mixture of venous and arterial blood. The arteries arising 

 from the ventricles are each provided wtth two valves. 



Portal system. — In amphibia and reptiles, as in fishes, there 

 are two lesser venous circulations; the one belongs to the liver, 

 and the other, which does not exist in birds or mammals, belongs 

 to the kidneys. In some reptiles ail the venous blood from the 

 posterior parts of the body is distributed to the liver and kidneys, 

 while in others a portion is sent to the inferior cava ; this latter 

 vessel also receives the venous blood which has circulated through 

 the kidneys. 



AVES. 



The heart, in this highly organised class of oviparous animals 

 consists of four separate and distinct compartments, and indeed pre- 

 sents a more perfect typical form than is met with in even the mam- 

 malia. Its form is conical, being sometimes short and wide, as in 

 the crane, and sometimes more elongated, as in the emeu. Its 

 situation is mesial, its axis parallel to that of the trunk, and in con- 

 sequence of the partial development of the diaphragm its apex rests 

 between the lobes of the liver. The right auricle seems consider- 

 ably larger than the left, and the auricular portion is distinctly 

 divided from the sinus by two semilunar muscular valves; one 

 attached along the anterior, and the other along the posterior side 

 of the sinus. The venous blood is returned to the sinus of the 

 auricle by an inferior and two superior venas cavae, and it is re- 

 markable that the left superior cava receives the coronary vein just 

 before its termination. The orifice of this vessel is opposite to that 

 of the inferior cava, but separated from it by a semilunar mem- 

 branous valve. The auriculo-ventricular opening is a small oblique 

 slit, and reflux from the ventricle is prevented by a thick strong 

 muscular valve so disposed as to give considerable impulse to the 

 flow of blood through the pulmonary artery. There is a small 

 muscular column at the upper part of the orifice, but it is only one 



