690 SPECIAL PHYSIOLOGY. 



Yertebrata, dependent upon the structure of their respiratory organs. Thus, 

 in Mammalia, Birds, Reptiles, and Amphibia, respiration is performed by 

 means of lungs, and hence they are called pulmonated or air-breathing Yerte- 

 brata. These consist of two divisions, one including Mammalia and Birds, 

 in which the temperature of the blood is high, and another, including Reptiles 

 and the perfect Amphibia, in which the temperature of the blood is low ; the 

 former are the warm-blooded, pulmonated or air-breathing, Yertebrata, and the 

 latter, the cold-blooded, pulmonated Yertebrata. Again, Fishes breathe by 

 means of gills or brancjiise, and constitute the branchiated or water-breathing 

 Yertebrata ; they are still more decidedly cold-blooded. In such of these divi- 

 sions, special modifications of the circulatory apparatus are met with, depend- 

 ent on differences in the degree, or kind, of their respiration. The Class 

 Amphibia contains animals which commence life as aquatic branchiated crea- 

 tures, but which, in the adult state, become pulmonated ; their circulatory 

 organs are accordingly modified during life, and in a few, which retain, in the 

 adult condition, both gills and lungs, the organs of circulation present a com- 

 posite form. In all cases, a portal circulation is present. 



In the warm-blooded, pulmonated Yertebrata, which include the Mammalia 

 and Birds, the heart, as in Man, consists of four cavities, two auricles, and two 

 ventricles, a right auricle and ventricle constituting the right side, and a left 

 auricle and ventricle forming the left side, of the heart, the two sides being 

 separated by a perfect intervening septum. The general distribution of the 

 vessels is also the same. All the blood which returns from the body to the 

 right auricle, is sent through the lungs, by the right ventricle, and on into the 

 left auricle, before it is again distributed to the body by the left ventricle. As 

 in the human body, there is a perfect double circulation. 



Certain minor peculiarities are met with. Thus, the position of the heart 

 is usually median, except in the orang-outang, and perhaps in other Anthro- 

 poid apes, in which it inclines to the left side, as in Man. In many Rumi- 

 nants, a bony structure, the bone of the heart, strengthens the base of the 

 ventricular septum. In the dugong, the apex of the heart is deeply notched, 

 the ventricles being there separate ; in the manatee, it is less notched. 



In Man and the higher Mammalia, the chief arteries spring from the arch of 

 the aorta unsymmetrically an arrangement said to favor the distribution of 

 blood to the right fore limbs, rather than to the left ; but in the lower Mam- 

 malia, the branches arise symmetrically. In the hind part of the tail of the 

 whale, the caudal artery runs, for purposes of protection, through the base of 

 ihe echelon bones, on the under side of the vertebrae. In many climbing 

 lemurs, in the lion and other large Carnivora, the brachial artery of the arm 

 passes through an opening in the humerus, and thus is protected from mus- 

 cular pressure ; so also does the artery of the coffin bone or hoof bone in the 

 liorse. 



A cluster of closely ramified arterial vessels, which frequently anastomose 

 together, and again coalesce into a single trunk, is named a rete mirabile. Ex- 

 amples of such retia mirabilia occur in the carotid arteries within the cranium 

 of the Ruminants, the effect of which may be to check the too rapid flow of 

 .blood to the brain, during grazing. Similar structures are found in the fore 

 limbs of the climbing sloths and lemurs. In diving animals, such as the Ceta- 

 -cea, large retia mirabilia exist in the thorax, the object of which is probably to 

 allow the prolonged suspension of respiration during the submergence of those 

 .animals. 



In the higher Mammalia, as in Man, there is but one superior vena cava ; 

 but in the Pachydermata and Rodentia, there are two superior venae cavse, 

 and sometimes a cross branch between them, in the neck. Suppose an enlarge- 

 ment of this cross branch, so as to form a left innominate vein, the blood from 

 the left side of the head and neck and the left upper limb would pass over to 

 the right side, whilst the trunk of the left vena cava might then be obliterated 

 as far as the heart. In certain diving Mammalia, venous plexuses or venous 

 retia mirabilia exist, in which the impure blood is for a time retained. The 

 portal system in the Mammalia, is entirely unconnected with the renal veins ; 

 .sometimes the portal veins have valves. 



In Birds, the heart is very strong, and lies exactly in the middle line ; more- 



