694 FUNCTIONS OP THE CEREBRO - SPINAL NERVOUS SYSTEM. 



only must there be a sufficient supply of blood, but it must make a regulated 

 pressure on the walls of the vessels. Now the Encephalon is differently cir- 

 cumstanced from other vascular organs, it being inclosed within an unyield- 

 ing bony case (275); and we find a special provision for equalizing the 

 bulk of the contents of this cavity, and for counterbalancing the results of 

 differences in the functional activity of the brain and in its supply of blood, 

 in the existence of a fluid which is found beneath the arachnoid, both on 

 the surface of the brain and spinal cord, and in the ventricles of the former. 

 The amount of this " cerebro-spiual fluid" seems to average about two 

 ounces; but in cases of atrophy of the brain, as much as twelve ounces of 

 fluid may sometimes be obtained from the crauio-spiual cavity; whilst in all 

 instances in which the bulk of the brain has undergone an increase, whether 

 from the production of additional nervous tissue, or from undue turgescence 

 of the vessels, there is either a diminution or a total absence of this fluid. 

 It appears from the experiments of Magendie (to whom our knowledge of 

 its importance is chiefly due), that its withdrawal in living animals causes 

 great disturbance of the cerebral functions, probably by allowing undue 

 distension of the bloodvessels; it is, however, capable of being very rapidly 

 regenerated ; and its reproduction restores the nervous centres to their nat- 

 ural state. As the cerebro-spiual fluid can readily find its way from the 

 subarachnoid spaces of the cranial cavity into those ot the spinal, and as it 

 is no less readily absorbed than reproduced, it evidently serves as an equalizer 

 of the amount of pressure within the cranial cavity; admitting the disten- 

 sion or contraction of the vessels to take place, within certain limits, without 

 any considerable change in the degree of compression to which the nervous 

 matter is subjected. That this uniformity is of the greatest importance to 

 the functional exercise of the brain, is evident from a few well-known facts. 

 If an aperture be made in the skull, and the protruding portion of the brain 

 be subjected to pressure, the immediate suspension of the activity of the 

 whole organ is the result; in this manner, a state resembling profound sleep 

 can be induced in a moment, the normal activity being renewed o-s momen- 

 tarily so soon as the pressure is withdrawn. 1 This phenomenon has often 

 been observed in the Human subject in cases in which a portion of the 

 cranial envelope has been lost by disease or injury. The various symptoms 

 of Cerebral disturbance which are due to a state of general Plethora, are 

 evidently owing to ail excess of pressure within the vessels; but an undue 

 diminution <>f pressure is no less injurious, as appears from the disturbance 

 in the Cerebral functions which results from the very opposite cause namely, 

 a depression of the power of the heart, or a deficiency of blood in the vessels. 

 It is of peculiar importance to bear in mind the disturbance of the Cerebral 

 functions occasioned by variations of internal prepare, when we are endeav- 

 oring to draw inferences from the phenomena presented by disease. 



5b'2. We shall now proceed with our Physiological inquiry into the func- 

 tions of the Cerebrum ; and shall appeal, as before, to Human and Com- 

 parative Anatomy, to Experiment, and to Pathology, for our chief data. 

 The anatomical relations of the Cerebrum to the other Encephalic centres, 

 clearly demonstrate that it is not one of the essential or fundamental por- 

 tions of the Nervous system ; but a snperadded organ, receiving all its im- 

 pulses to action from the parts below, and operating upon the body at large 

 through them. And its great bulk, joined to its position at the summit of 

 the whole apparatus, the vesicular substance of its convolutions affording 

 a termination to the fibres in connection with it, and not being for the most 



1 Schiff states that tli<> Indian snake-charmers are accustomed to produce rigidity 

 of the body in these animals by pressure on the occiput. 



