THE BLOODVESSELS g 7 



If the brain be exposed it is observed to rise and fall synchron- 

 ously with the respiratory movements. Expiration causes the brain 

 to rise by hindering the return of blood, while inspiration causes 

 it to fall by facilitating its flow. Owing to the incompressibility 

 of the brain substance in the cranial cavity the pulse is trans- 

 mitted through the brain substance to the veins, and causes 

 the blood to issue from them in pulses synchronous with the 

 arterial pulses. 



There is no vasomotor nerve supply proved to exist in the 

 brain. 



The singular arrangement of the venous plexuses of the corpus 

 cavernosum penis admits of this organ attaining a great increase 

 in size, a condition which in the brain every measure is adopted 

 to prevent. The considerable size of the venous plexuses of the 

 penis, their frequent intercommunication, the muscular pressure 

 to which the veins leading from the sinuses are exposed, produce 

 under the direction of the vasomotor nervous system a con- 

 siderable increase in the volume of the part. 



In some other organs the distribution of the bloodvessels is 

 also peculiar. It is not known why the spermatic artery and 

 plexus of veins should take such a remarkably tortuous course. 

 Possibly, in some way or other, it may be concerned with the 

 secretion of the glands, but its use is far from clear. On the 

 other hand, tortuous vessels in the walls of hollow viscera, such 

 as the stomach and intestines, perform a very evident function. 

 We have only to think of the size of a collapsed and full stomach 

 in the horse to recognise the necessity for some arrangement 

 existing to prevent over-stretching of the vessels or interference 

 with the blood supply. 



The vast venous and arterial plexuses of the foot of the horse 

 are a peculiarity in the circulation dealt with in the chapter 

 devoted to the Foot. 



Pathological. 



A man is considered to be as old as his arteries, but a horse is as 

 old as his feet and legs. 



It is a remarkable fact that very little of the hard life of a horse 

 falls on his arteries ; with age the vessels become more rigid, but no 

 sudden strain produces aneurisms, such as might be expected from 

 the class of work performed ; this is probably due to the fact that he 

 does not suffer from syphilis. There is, however, one kind of strain 

 which arises in the hunting field, or under similar circumstances, 

 in which the walls of the external and internal iliac arteries suffer ; 

 in consequence of this a thrombus forms in the vessels which become 

 partly or completely obliterated. Collateral circulation suffices in 

 a state of repose, during which not a sign of any circulatory trouble 

 is evident, but as soon as the animal gets to work sudden and painful 



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