The Bloodvessels. 73 



diary ones in the spinal cord, there is reason to believe that 

 local centres exist which, so long as they are in connection 

 with the medulla, are subordinate to it, but may, when 

 separated from it, act as centres on their own account ; 

 this latter condition being abnormal, it is probable that the 

 local centres are for the purpose of keeping up a closer 

 touch with those in the cord and medulla, and so main- 

 taining the normal vascular tone. 



Peculiarities in the circulation through various parts 

 occur as the result of their special function ; they are 

 observed in the brain, erectile tissues, etc. The great 

 vascularity of the brain necessitates that the blood should 

 pass to it with a degree of uniformity which will ensure 

 the carrying out of its functions. We see this provided 

 for in the frequent arterial anastomoses — for example, 

 the Circle of Willis and the Rete Mirabile of ruminants, 

 which ensures that not only does the blood enter with 

 diminished velocity, but that if a temporary obstruction 

 occurs in one vessel its work is readily performed by the 

 others. The rete mirabile alluded to, which forms the beauti- 

 ful arterial plexus on the base of the brain of ruminants, is 

 considered by some to regulate the flow of blood to the brain 

 while the head is depressed during grazing, and that it 

 accounts for the absence of cerebral hemorrhage in these 

 animals. It is probable that this may be one of its 

 functions ; but the horse possesses no rete, and his head is 

 depressed during grazing for more hours out of the twenty- 

 four than is the case with ruminants. It has probably, 

 therefore, some other function to perform. 



The manner in which the vessels break up in the pia 

 mater before entering the brain substance, is also another 

 arrangement for equalizing the distribution of the blood. 

 The venous arrangement of the brain is very remarkable ; 

 the walls of the veins are composed of layers of the dura 

 mater, and even portions of the cranial bones may enter 

 into their formation. The veins or sinuses of the brain 

 are large cavities which, from their arrangement, are most 

 unlikely to suffer from compression, and from the rigidity 



