64 GENERAL ANATOMICAL CHARACTERS 



each guarded by three semilunar valves. The aorta is single, and 

 arches over the left bronchial tube. After supplying the tissues of 

 the heart itself with blood by means of the coronary arteries, it 

 gives off large vessels ("carotid") to the head and ("brachial") to the 

 anterior extremities. The mode in which these vessels arise from 

 the aorta varies much in different mammals, and the study of their 

 disposition affords some guide to classification. In nearly all cases 

 the right brachial and carotid have a common origin (called the 

 "innominate artery" in anthropotomy). The other two vessels 

 may come off from this, as is the rule in Ungulates, the common 

 trunk constituting the " anterior aorta " of veterinary anatomy ; or 

 they may be detached in various degrees, both arising separately 

 from the aorta, as in Man, or the left carotid from the innominate 

 and the left brachial from the aorta, a very common arrangement ; 

 or the last two from a common second or left innominate, as in 

 some Bats and Insectivores. The aorta, after giving off the inter- 

 costal arteries, passes through the diaphragm into the abdomen, and, 

 after supplying the viscera of that cavity by means of the gastric, 

 hepatic, splenic, mesenteric, renal, and spermatic vessels, gives off 

 in the lumbar region a large branch (iliac) to each of the hinder 

 extremities, which also supplies the pelvic viscera, and is continued 

 onwards in the middle line, greatly diminished in size, along the 

 under surface of the tail as the caudal artery. In certain mammals, 

 arterial plexuses, called retia mirabilia, formed by the breaking up 

 of the vessel into an immense number of small trunks, which may 

 run in a straight course parallel to one another (as in the limbs of 

 Sloths and Slow Lemurs), or form a closely packed network, as in 

 the intracranial plexuses of Euminants, or a sponge-like mass of 

 convoluted vessels, as in the intercostals of Cetaceans, are 

 peculiarities of the vascular system the meaning of which is 

 not in all cases clearly understood. In the Cetacea they are ob- 

 viously receptacles for containing a large quantity of oxygenated 

 blood available during the prolonged immersion, with consequent 

 absence of respiration, to which these animals are subject. 



The vessels returning the blood to the heart from the head and 

 upper extremities usually unite, as in Man, to form the single vena 

 cava superior or precaval vein, but in some Insectivores, Chiroptera, 

 and Rodents, in the Elephant, and all Marsupials and Monotremes, 

 the two superior caval veins enter the right auricle without uniting, 

 as in birds. In Seals and some other diving mammals there is a 

 large venous sinus or dilatation of the inferior vena cava immediately 

 below the diaphragm. In the Cetacea the purpose of this is supplied 

 by the immense abdominal venous plexuses. As a rule the veins 

 of mammals are furnished with valves, but these are said to be 

 altogether wanting in the Cetacea, and in the superior and inferior 

 cava, subclavian and iliac veins, the veins of the liver (both portal 



