74 A Manual of Veterinary Physiology. 



of their walls are not capable of bulging as most veins do 

 when obstructed ; in this way the return of the venous 

 blood is provided for. 



The pulsations observed in the exposed brain are not 

 due to the pulse in the arteries of the brain, but arise from 

 the respiratory movements : expiration causes the brain to 

 rise by hindering the return of blood, whilst inspiration 

 causes it to fall by facilitating the return of blood. 



The cerebral circulation is considerably assisted by the 

 presence of fluid within the ventricle of the brain. This 

 fluid readily passes from ventricle to ventricle, and from 

 ventricle to spinal cord : in this way, as the external pressure 

 becomes greater the internal becomes less, and so com- 

 pression of the brain substance is avoided. 



The value of these provisions in the case of draught 

 horses with tightly-fitting collars is very evident ; as a 

 practical fact a tight collar pressing on the root of the 

 neck rarely produces any cerebral symptoms ; but this is 

 probably due to the natural provision existing to prevent 

 enlargement of the veins or considerable blood pressure on 

 the brain. 



The singular arrangement of the venous plexuses of the 

 corpus cavernosum penis, admits of this organ attaining a 

 condition which in the brain every measure is adopted to 

 prevent — viz., avast increase in size. The enormous size of 

 these venous plexuses, their frequent intercommunication, 

 the muscular pressure to which the veins leading from the 

 sinuses are exposed, produce a considerable increase in the 

 volume of the organ under the direction of the vaso-motor 

 nervous system. 



In some organs the vascular arrangement is peculiar, 

 and probably depends on the function of the part. It is 

 not clear, however, why the spermatic artery and plexus of 

 veins should possess such a remarkably tortuous course. 

 Doubtless, in some way or other, it is concerned with the 

 secretion of the glands with which the vessels are connected, 

 but its use is far from clear. On the other hand, tortuous 

 vessels in the walls of the hollow viscera, such as the 



