NERVOUS CENTRES. (HUMAN ANATOMY. THE CEREBRO-SPINAL FLUID.) 641 



the authority of Cruveilhier, that in fifteen sub- 

 jects in which the latter anatomist found this 

 orifice, its margins had the torn appearance in 

 every one; " that around the opening, here 

 and there, there existed the debris of mem- 

 branes."* 



My own opinion is that this orifice does not 

 exist naturally, but that it is produced by the 

 violence to which the brain is subject in its 

 removal, or in the manipulations necessary for 

 demonstrating it. It appears to me that the 

 fourth ventricle is closed in the same way as 

 the inferior horn of the lateral ventricles, namely, 

 by the reflection of its proper membrane from 

 its floor on to the adjacent pia mater. This 

 membrane is so extremely delicate that the 

 slightest traction upon it is sufficient to dis- 

 turb its connexions. Its existence may be 

 best proved by the resistance which a probe 

 pushed into the fourth ventricle from above 

 experiences at its inferior extremity, a resist- 

 ance, however, which a little force can over- 

 come. Or, if the fourth ventricle be opened 

 from the side, by a vertical section of the 

 median lobe of the cerebellum some dis- 

 tance to one side of the median plane, and 

 if this be done on a brain previous to its re- 

 moval from the body, or on one which has been 

 removed with great caution, so as to occasion 

 the least possible disturbance to the parts, it 

 will be found that the ventricle is closed below 

 by the reflection of its proper membrane upon 

 the pia mater. There can be no doubt that 

 fluid driven against this membrane with foice 

 would easily rupture it, whether from with- 

 in the ventricle or from the sub-arachnoid 

 space.f 



It is plain that if there be a direct commu- 

 nication between the fluid in the ventricles and 

 that in the sub-arachnoid cavity at the inferior 

 extremity of the fourth ventricle, it must take 

 place through an opening in that portion of the 

 pia mater which ascends into the fourth ven- 

 tricle to form the choroid plexus. But it is 

 not necessary to have recourse to such a sup- 

 position to account for the transmissibility of 

 fluid from one cavity to the other, for the 

 pia mater is evidently hygrometric, and will 

 readily admit of the passage of fluid through 

 it by endosmose, and it is highly probable 

 that, if any interchange of fluid takes place 

 between the mtra-ventricular cavity and the 

 sub-arachnoid space, it is accomplished through 

 the influence of endosmose and exosmose, 

 effected not merely by the pia mater at the 

 inferior extremity of the fourth ventricle, but 

 likewise by that at the inferior cornua of the 

 lateral ventricles, and perhaps also by that of 

 the third ventricle, at the velum interpositum. 

 And it is worthy of remark, as tending to con- 

 firm this opinion, (which, so far as I am aware, 

 has not previously been suggested,) that at 

 each of these situations there is a conflux (to 

 use Majendie's phrase) of the sub-arachnoid 

 fluid. 



* Martin St. Ange. Sur les membranes du 

 cerveau et de la moelle epiuiere. 



t See the description of the fourth ventricle fur- 

 ther on. 



VOL. III. 



Cruveilhier lays some stress upon the fact that 

 in apoplexy the blood escapes from the ventricle 

 into the sub-arachnoid space. For my own part, 

 I would say that this occurrence takes place as 

 often, if not more frequently, at the inferior cor- 

 nua of the lateral ventricles, as at the fourth ven- 

 tricle. And therefore, if such a fact be used as 

 an argument in favour of the direct communica- 

 tion of the latter with the sub-arachnoid space, it 

 ought equally to lead to the supposition of the 

 existence of similar orifices at the former situa- 

 tions, the absence of which may be easily proved. 

 Moreover it may be stated that blood sometimes 

 extravasates into the arachnoid sac, breaking 

 through the arachnoid membrane; it is, there- 

 fore, less difficult to conceive its bursting the 

 pia mater, which is evidently more porous, and 

 is the seat of those vessels from which the he- 

 morrhage comes, a morbid condition of which 

 is the frequent precursor of the apoplectic 

 attack. 



The best way of obtaining the sub-arachnoid 

 fluid with a view to form an estimate of its 

 quantity, is to open the dura mater and arach- 

 noid in the lumbar region of the spine, having 

 previously, by means of a trephine, made a 

 small perforation in the skull, so as to allow the 

 pressure of the atmosphere to bear upon the 

 cranial contents. " If," says Cotunnius, " you 

 open the vertebrae of the loins before the head 

 is touched, and cut the enclosed tube of the 

 dura mater, a great quantity of water will burst 

 out, and after all this spontaneous flux of water 

 is spent, if you lift up the head, and shake it 

 toward the aperture, a more plentiful stream 

 will burst out, as if a new fountain was un- 

 locked. In these experiments, which I made 

 on the bodies of near twenty adults, and which 

 I repeated at different times, I could draw off 

 freely from the hollow of the spine four and 

 sometimes even five ounces of water: I com- 

 monly found it very clear in such subjects, 

 although it sometimes inclined a little to a 

 yellow colour; but in fetuses strangled in 

 difficult labour, little as it was, I observed 

 it to be always red and opaque."* 



The estimate of the quantity of sub-arach- 

 noid fluid here assigned by Cotunnius exceeds 

 that which Majendie deduces from his expe- 

 riments, who states that in general in a subject 

 of adult age and mean size, and in moderate 

 condition, two ounces may be regarded as the 

 minimum quantity. Much depends upon the 

 age and size of the subject and the state of nu- 

 trition of the nervous centres. In children the 

 quantity is very small ; in old age, when the 

 brain and spinal cord have shrunk considerably, 

 the quantity is large. In examining the bodies 

 of the aged poor, as Majendie remarks, eight, 

 ten, or twelve ounces of fluid may be obtained 

 from the cranio-spinal cavity, according as 

 there is a greater or less degree of atrophy of 

 the brain. 



In judging of the quantity of fluid around 

 as well as within the cerebro-spinal centres, 



* From an English translation of Cotunnius's 

 essay, entitled, A Treatise on the Nervous Sciatica, 

 or Nervous Hip Gout, translated by Henry Crantz, 

 London, 1775. 



2 T 



