CIRCULATION IN LOWEK ANIMALS. 468 



Tlie right auricle is mucli larger than the left, and a single triangular 

 valve takes the place of the tricuspid. 



The common aorta divides into three branches, a single descending or 

 posterior aorta, and a right and left trunk, termed by Chauveau the bracMo- 

 cqjhalic arteries, the first supplying chiefly the viscera ; the latter the wing, 

 head, neck, and anterior parietes of the body ; these lateral trunks are regarded 

 by some authorities as representing the mammalian thoracic arteries, giving off 

 the carotid and subclavian branches. 



A large number of retia mirabilia are found among the arteries of the head 

 and viscera. 



There is only one terminal indmonary vein and two superior cava. The 

 jugular veins have a large transverse anastomotic branch just below the cranium. 

 The fortal vein receives branches from the veins of « 



the posterior extremities, notably the coccygeo- 

 niesenteric vein, which is connected with the femoral 

 by the renal-portal vein. Thus is constituted the 

 renal-jyorfal si/stem, whose existence in the bird is a 

 disputed point, even amongst high authorities. 



The blood of birds is higher in temperature than 

 that of any other vertebrate class. The corpuscles 

 are large, elliptical in shape, and nucleated. 



The lymphatics consist of glands and vessels. 

 There are two thoracic ducts, each discharging into 

 the cava of its own side. They originate in a kind 

 of receptaculum chyli, near the coeliac axis. The 

 glands are said to be confined to the cervical region. 



CIRCULATION IN LOWER ANIMALS. 



It may be not out of place here to explain A'ery 

 briefly how the circulatory apparatus, in animals Diagram°ofthe Circu- 

 placed lower than the class Aves, differs in the lation'in a Keptile. a, 

 general plan from the type Ave have hitherto con- I'uiwonic, and h, Soma- 

 sidered. It will be seen that in none of the mammals slloXg"'le°three"cavi- 

 or birds we have attempted to describe, is tlieie any ties. The arrows indicate 

 important dift'erence in the general plan of the heart, the course of the blood 

 course of the blood, or function of the vessels. current. 



In that class placed next below the Aves, the Beptilia, or reptiles, we find 

 the following interesting features (see Fig. 171). The heart is less complex, 

 having only three cavities, two auricles, and one common ventricle. As the 

 general disposition of the vessels is the same as in the heart of four cavities, it 

 follows that both the purified blood from the lungs, and the venous blood from 

 the cava3, are passed through their respective auricles to the ventricle, where they 

 become mixed. This mixed blood is driven into the aorta and pulmonary artery, 

 both of which arise from the common ventricle. Hence, though the circulation 

 in these animals is double, the blood supplied to the tissues is impure or mixed. 



In a still lower class of animals, that of the Pisces, or fishes, we find that the 

 heart is still more rudimentary, consisting of but two cavities, an auricle and 

 a ventricle, but still the tissues are supplied with pure arterial blood ; the 

 course of the blood being as follows : — The venous blood is conveyed bv the 



