64 GENERAL ANATOIMHCAL CHARACTERS 
each guarded by three semilunar valves. ‘Tho aorta is single, and 
arches over tho left bronchial tube. After supplying tho tissues of 
the hoart itself with blood by means of tho coronary arterios, it 
gives off largo vessels (“carotid”) to the head and (“brachial”) to the 
anterior extremities. Tho mode in which these vessols arise from 
the aorta varies much in different mammals, and tho study of their 
disposition affords somo 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 tho “anterior aorta” of veterinary anatomy ; or 
they may be detached in various degrees, both arising separately 
from the aorta, as in Man, or tho left carotid from the innominate 
and the left brachial from the aorta, a vory common arrangement ; 
or tho last two from a common second or left innominate, as in 
some Bats and Insectivores. Tho aorta, after giving off the inter- 
costal arteries, passes through tho diaphragm into the abdomen, and, 
after supplying the viscora of that cavity by moans of tho gastric, 
hepatic, splenic, mesontoric, renal, and spermatic vessels, gives off 
in the lumbar region a large branch (iliac) to each of tho hindor 
extremities, which also supplies the polvic viscera, and is continued 
onwards in tho middlo line, greatly diminished in sizo, along the 
under surface of tho tail as the caudal artery. In certain mammals, 
arterial ploxusos, called retin mirahilia, 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 Ruminants, or a sponge-like mass of 
convoluted vessels, as in the intercostals of Cetaceans, are 
peculiarities of tho vascular system the meaning of which is 
not in all cases clearly understood. In the Cetacea they aro ob- 
viously receptacles for containing a largo quantity of oxygenated 
blood available during the prolonged immersion, with consequent 
absence of respiration, to which those animals are subject. 
The vessols returning the blood to the heart from the head and 
upper extremities usually unite, as in Man, to form tho single vena 
curd superior or procaval voin, but in somo Inscctivores, Chiroptora, 
and Rodents, in the Elophant, and all Marsupials and Monotromos 
the two suporior caval veins enter tho right auricle without uniting, 
as in birds. In Scals and somo other diving mammals there is a 
large venous sinus or dilatation of the inforior vena cava immediately 
below tho diaphragm, — In the Cotacoa the purposo of this is supplied 
by tho immenso abdominal venous ploxusos. As a rule tho voins 
of mammals aro furnished with valves, but those are said to he 
utogothor wanting in the Cetacoa, and in the suporior and inferior 
cava, subclavian and ilive veins, the veins of the liver (both portal 
