230 PECULIARITIES OF DISTRIBUTION OF ARTERIES. 
originate the trunks which supply the brain in the usual 
manner. The object of this apparatus appears to be, to pre¬ 
vent the influence of gravitation from causing a too great rush 
of blood towards the brain, when the head is in a depending 
position; for the rapidity of its flow will be checked, as soon 
as it enters the network, and is distributed through its 
numerous canals. A similar conformation is found in the 
blood-vessels of the limbs of the Sloth, and of some other 
animals which resemble that animal in the sluggishness of 
their movements; and its object is probably to prevent the 
muscles from receiving too rapid a supply of blood, which 
would give them what (for these animals) would be an undue 
energy of action; whilst, by the very same delay, their power 
of acting is greatly prolonged,—as we find it to be in Beptiles, 
whose circulation is languid (§ 284). 
265. In the Whale tribe, and some other diving animals 
that breathe air, we find a curious distribution of the blood¬ 
vessels, which has reference to their peculiar habits. The 
intercostal arteries (which are sent-off from the aorta to the 
spaces between the ribs on each side) are enormously dilated, 
and are twisted into thousands of convolutions, which are 
bound together into a mass by elastic tissue. This mass, 
which is of considerable bulk, lies at the back of the chest, 
along both sides of the vertebral column ; and it serves as a 
reservoir, in which a great quantity of arterial blood may be 
retained. The veins also have very large dilatations, which 
are capable of being distended, so as to hold a considerable 
amount of venous blood ; and thus, while the animal is pre¬ 
vented from breathing by its submersion in the water, the 
circulation through the capillaries of the system is sustained, 
by the passage of the blood stored up (as it were) in the 
arterial system, into the venous reservoirs. If this provision 
did not exist, the whole circulation would come to a stand, 
in consequence of the obstruction it meets with in the lungs, 
when the breathing is stopped. 
266. With regard to the Venous system , there is little to be 
added to what has been already stated (§§ 248-250) as to its 
general character and distribution. The large proportion which 
its capacity bears to that of the arterial system, is shown by 
the fact, that every main artery is accompanied by a vein (fre¬ 
quently by two) considerably larger than itself; and that the 
