THE VASCULAR ARRANGEMENTS OF THE BRAIN. 737 



vessels. It would be hazardous to insist too much on this negative result, 

 especially since the observations have been chiefly directed to the nerves of 

 the neck, the experimental difficulties of investigating the presence of vaso- 

 motor fibres in the cranial nerves being very great. Still it may be urged 

 and indeed has been urged that the flow of blood through the brain is so 

 delicately responsive to the working of the general vasomotor mechanism 

 just because it has no vasomotor nerves of its own. In such an organ as 

 the kidney, an increase of general blood-pressure, as we have more than 

 once insisted, may or may not lead to a greater flow through the kidney 

 according as the vessels of the kidney itself, through the action of the renal 

 vasomotor nerves, are dilated or constricted ; and as we have seen, a con- 

 striction of the renal vessels may be one of the contributors to the increased 

 general pressure. In the brain, on the other hand, an increase of general 

 arterial pressure seems always to lead to increase of flow. Thus in the 

 Traube-Hering undulations just mentioned, the expansions of the brain 

 are coincident with the rises of the general pressure, whereas in the normal 

 kidney and in other organs the local Traube-Hering undulation reverses the 

 general one, the shrinkings are synchronous with the rises of pressure, the 

 local constriction being one of the factors of the general rise. It is argued 

 that in the absence of vasomotor nerves of their own, the cerebral vessels 

 are wholly, so to speak, in the hands of the general vasomotor system, so 

 that when the blood-pressure is high owing to a large vaso-constriction in 

 the abdominal viscera, more blood must necessarily pass to the brain, and 

 when again the blood-pressure falls through the opening of the splanchnic 

 flood-gates ( 159) less blood necessarily flows along the cerebral vessels. 

 And indeed one may recognize here a sort of self-regulating action ; for 

 diminishing the supply of blood to the vasomotor centre in the bulb acts, as 

 we know, as a powerful stimulus in producing vaso-constriction, and so leads 

 to a rise of blood-pressure ; but this very rise of blood-pressure drives more 

 blood to the brain, including the bulb, and thus the injurious effects to the 

 brain threatened by an anaemic condition are warded off by the very be- 

 ginning of the anaemia itself. All these advantages are, however, quite 

 compatible with the coexistence of special vasomotor mechanisms. 



613. Moreover, the flow of blood to, and consequent change in the bulk 

 of, the brain, and indeed the flow of blood through the brain, as measured 

 by the venous outflow, may be modified independently of changes in the 

 general blood-pressure. For instance, stimulation of the motor region of the 

 cortex quickens the venous outflow, without producing any marked change 

 in the general blood-pressure ; this feature becomes very striking at the onset 

 of epileptiform convulsions when these make their appearance. It is diffi- 

 cult not to connect such a result of functional activity with some special 

 vasomotor nervous arrangement comparable to that so obvious in the case 

 of a secreting gland. Again, it has been observed that certain drugs have 

 an effect on the volume of the brain, quite incommensurate with their effect 

 on the vasomotor system ; thus in particular the injection into the general 

 blood-stream of a weak acid produces a large and immediate expansion of 

 the brain, while the introduction of a weak alkali similarly gives rise to 

 similar considerable shrinking. It is suggested that these effects are pro- 

 duced by the acid or alkali acting directly on the muscular coats of the 

 minute arteries and so leading to relaxation or contraction respectively. In 

 treating of the chemistry of nervous substance ( 70) we stated that "the 

 gray matter of the central nervous system is said to be slightly acid during 

 life and to become more acid after death." Recent observations go to show 

 that the gray matter of the cortex is faintly alkaline during life and under 

 normal conditions, but becomes acid after death or when its blood-supply is 



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