320 PHYSIOLOGY. 



If the pressure in the large veins increases, whilst that of the 

 aorta is the same, then there is more blood in the cranial veins, less in 

 the cranial arteries and a slowing of the rapidity of circulation in the 

 brain. If the pressure in the aorta is elevated and that in the large 

 veins remains the same, then the cerebral arteries contain more blood, 

 the veins less and the rapidity of the cerebral circulation is accelerated. 

 Howell found that a rise of pressure, however great, in the cerebral 

 arteries does not cause directly any impediment to the blood-flow, either 

 temporarily or permanently. The circulation in the brain behaves 

 in this respect as it does in other organs of the body : the greater the 

 arterial pressure the more abundant is the flow of blood, and temporary 

 anaemia can not be produced in this way. 



The existence of vasomotor fibers in the brain is still a subject of 

 debate. 



VASOMOTOR NERVOUS SYSTEM. 



Thus far the circulatory system, except the heart, has been con- 

 sidered almost entirely from its physical standpoint: that it is a 

 system of more or less elastic tubes through which the blood is pro- 

 pelled by the action of the heart. There was considered the resist- 

 ance which its passage met with, the pressure exerted by this vital 

 fluid, together with the interpretations and the physical causes for 

 variations in each function or property. It yet remains to consider 

 that they are living tubes, and that they and the heart are kept in a 

 very delicate balance by reason of certain physiological mechanisms. 

 The agents governing their functions are impulses that emanate 

 from the central nervous system via certain nerves. The circula- 

 tory apparatus, as every other system, or organ, or part of the entire 

 economy, is under one management and direction located within the 

 central nervous system. It is this latter system that, by the main- 

 tenance of its functions, produces harmony and division of labor 

 throughout the entire body. 



It has been previously stated that the musculature of the heart 

 is under the guidance of two sets of nerve-fibers : one set to restrain 

 heart-action; another to increase it. Likewise there are two sets of 

 fibers which supply the musculature of the vessels (particularly the 

 arterioles, since their proportionate quantity of circular, unstriped 

 muscular fibers is greatest), which, together with their centers, con- 

 stitute the vasomotor system. 



The vasomotor system may be said, then, to be composed of the 

 vasomotor center, situated in the medulla, together with some acces- 



