THE CIRCULATION OF THE BLOOD 



paratively empty when the animal was kept suspended by the ears. Thus 

 although the total volume of the contents of the cranium is probably nearly 

 always the same, yet the quantity of blood in it is liable to variation, its in- 

 crease or diminution being accompanied by a simultaneous diminution or 

 increase in the quantity of the cerebro-spinal fluid. The cerebro-spinal 

 fluid being readily removed from one part of the brain and spinal cord to 

 another, and capable of being rapidly absorbed and as readily effused, would 

 serve as a kind of supplemental fluid to the other contents of the cranium 

 to keep it uniformly filled. Although the arrangement of the blood-vessels 

 insures to the brain an amount of blood which is tolerably uniform, yet with 

 every beat of the heart, and every act of respiration, and under many other 

 circumstances, the quantity of blood in the cavity of the cranium is con- 

 stantly varying. 



The brain, however, is entirely dependent upon the general blood pres- 

 sure for variations in the quantity of blood which it receives. During a 

 high blood pressure the amount of blood that flows in a given unit of time is 

 greater and during low blood pressure less. Howell has shown that in the 

 decapitated dog's brain the flow of blood is directly proportional to the differ- 

 ence in pressure. Numerous attempts have been made to show vaso-motor 

 mechanisms for the cerebral arteries, but without success. Huber has shown 

 nerve endings in such arteries by histological methods. Bayless, Hill, and 

 Gulland make the statement that " no evidence has been found of the exist- 

 ence of cerebral vaso-motor nerves, either by means of stimulation of the 

 vaso-motor center or central end of the spinal cord, after division of the cord 

 in the upper dorsal region, or by stimulation of the stellate ganglion, and 

 that is to say the whole sympathetic supply to the carotid and vertebral 

 arteries." Vaso-motor regulation of the flow of blood through the brain 

 can be accomplished only by the indirect mechanism of regulation of general 

 blood pressure through variations in the heart's activity, or through the 

 effects of vaso-constrictions or dilatations in large areas other than the brain. 



The Vascular Nerves for the Thoracic Viscera. Numerous efforts 

 have been made to determine the vaso-motor nerve supply for the thoracic 

 organs, the heart and lungs. In the heart the observation is rendered com- 

 plex by the fact of the rhythmic contractions which produce mechanical 

 pressure on the coronary arteries. Martin, by direct observation through a 

 lens, and Porter, by measuring the outflow of the coronaries upon vagus 

 stimulation, came to exactly opposite views; the former that the vagus con- 

 tained vaso-dilators, the latter that it contained vaso-constrictors. Still 

 other experiments have been made to prove either constrictor or dilator 

 nerves for the coronary arteries. 



The lesser circulation through the lungs has also proven a difficult situa- 

 tion to interpret as regards any nervous regulation of the pulmonary arterioles. 

 The evidence, while not conclusive, is that the vaso-constrictor supply to the 



