CEPHALO-SPINAL FLUID. 109 



On dissection, we find that the encephalon fills the cavity of the cra- 

 nium; during life, therefore, it must be pressed upon, more or less, by 

 the blood in the vessels, and by the serous fluid exhaled by the pia mater 

 into the subarachnoid tissue. Thence it penetrates into the ventricles, 

 according to M. Magendie, at the lower end of the fourth* ventricle, 

 at the calamus scriptorius. The quantity varies according to the age 

 and size of the patient, and usually bears an inverse proportion to the 

 size of the encephalon. It is seldom, however, less than two ounces, 

 and often amounts to five. M. Magendie is of opinion, that the fluid is 

 secreted by the pia mater, and states, that it may be seen transuding 

 from it in the living animal. The results of chemical analysis appear 

 to show, that it differs from mere serum. It is obviously, however, 

 almost impracticable if not wholly so to separate the consideration 

 of this fluid from that met with in the cavity of the arachnoid. 



The spinal marrow does not, as we have seen, fill the vertebral canal; 

 the cephalo-spinal fluid exerts upon it the necessary pressure; added to 

 which, the pia mater seems to press more upon this organ than upon 

 the rest of the cerebro-spinal system. A certain degree of pressure 

 appears, indeed, necessary for the due performance of its functions; 

 and if this be either suddenly and considerably augmented, or dimi- 

 nished, derangement of function is the result. M. Magendie, 1 however, 

 asserts, that he has known animals, from which the fluid had been re- 

 moved, survive without any sensible derangement of the nervous func- 

 tions. It is this fluid, which is drawn off by the surgeon when he 

 punctures in a case of spina bifida. 



When the brain is examined in the living body, it exhibits properties, 

 which, some years ago, it would have been esteemed the height of hardi- 

 hood and ignorance to ascribe to it. The opinion has universally pre- 

 vailed, that 'all nerves are exquisitely sensible. Many opportunities 

 will occur for showing, that this sentiment is not founded on fact ; even 

 the encephalon itself, the organ in which perception takes place, is 

 insensible, in the common acceptation of the term ; that is, we may 

 prick, lacerate, cut, and even cauterize it, yet no painful impression 

 will be produced. Experiment leaves no doubt regarding the truth of 

 this, and we find the fact frequently confirmed by pathological cases. 

 Portions of brain may he discharged from a wound in the skull, and 

 yet no pain 'be evinced. In his "Anatomy and Physiology," Sir C. 

 Bell 2 remarks, that he cannot resist stating, that on the morning on 

 which he was writing, he had had his finger deep in the anterior lobes 

 of the brain ; when the patient, being at the time acutely sensible, and 

 capable of expressing himself, complained only of the integument. A 

 pistol-ball had passed through the head, and Sir Charles, having ascer- 

 tained, that it had penetrated the dura mater by forcing his finger into 

 the wound, trephined on the opposite side of the head, and extracted it. 



By the experiments, instituted by MM. Magendie, 3 Flourens and 



I Precis Elementaire, seconde edit., i. 192; and Recherches Physiologiques et Cliniques 

 sur le Liquide Cephalo rachidien cm Cerebro spinal, Paris, 1842. 



II Fifth Airier, edit, by J. D. Godman, ii. 6, 1827. 

 3 Precis Elementaire, i. 325. 



