CIRCULATION IN LOWER ANIMALS. 



463 



The right auricle is much larger than the left, and a single triangular 

 valve takes the place of the tricuspid. 



The common aorta di"\'ides into three branches, a single descending or 

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

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

 head, neck, ai d 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 pulimonary vein and two superior cavce. The 

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

 The portul vein receives branches from the veins of the posterior extremities,^, 

 notably the coccygeo-mesenteric vein, which is con- a 



nected with the femoral by the renal-portal vein. 

 Thus is constituted the renal-portal system, 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. 



Fia. 171. 

 Diagram of the Circu- 

 lation in a Reptile, a, 

 Pulmonic, a.nd h, Soma- 

 tic circulation ; c, Heart, 

 showing the three cavi- 

 ties. The arrows indicate 

 the course of the blood 

 current. 



ft may be not oat of place here to explain very 

 briefly how the circulatory apparatus, in animals 

 placed lower than the class Aves, differs in the 

 general plan from the type we have hitherto con- 

 sidered. It will be seen that in none of the mammals 

 or birds we have attempted to describe, is there any 

 important difference in the general plan of the heart, 

 course of the blood, or function of the vessels. 



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 ca^ae, 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 



