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 Eeptiles, 

 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 



