That fluid exists in the ventricles of the brain has long been known to anatomists ; and it seems highly probable that this fluid is se creted by the processes of pia mater which are found in all these cavities; or possibly by the membrane which lines their surface. Does the internal fluid communicate with that in the sub-amchnoid space ? Majendie affirms that a communication takes place by means of an open ing which is situated at the inferior extremity of the fourth ventricle. I have not been able to satisfy myself of the existence of such an open ing; the following is AIajendie's description of it : " The true orifice, constant and normal, by which the cerebro-spinal fluid continually passes, either to enter the ventricles or to issue from them, may be seen at the inferior termination of the fourth ventricle, at the place named le bec de la plume' by the old anato mists.
" To demonstrate the existence of this ori fice it is necessary to raise up, and to separate slightly from one another, the lobules of the inferior vermiform process of the cerebellum, and without breaking any of the vascular ad hesions which unite this part of the cerebellum with the spinal pia mater, we perceive the angular excavation which terminates the fourth ventricle. Its surface is smooth, even (polie), and is prolonged as far as the ventricle of the cerebellum. Such is the anterior part of the orifice: the lateral and superior parts are formed by the choroid plexuses of the organ and by a homy medullary lamella (valve of Tann), the extent of which is variable, and which adheres to the prominent border of the fourth ventricle. The form and dimensions of the opening vary with the individual, and with the quantity of cerebro-spinal fluid, so that when the latter exists in considerable quantity the opening, can admit the extremity of a finger. Most fre quently, when the quantity of the liquid is normal, the orifice does not exceed two or three lines in diameter in every direction, but it is frequently subdivided by vessels which pass from the medulla oblongata to the cerebel lum. Sometimes the orifice is restricted by one or by both of the posterior cerebellar ane ries which pass across it." Such is the description of the orifice to which Majendie has given the high-sounding title " Orifice des «with eneephaliquis." lle states that when fluid is injected into the spinal sub-arachnoid cavity, it makes its way into the ventricles of the brain through this orifice, a statenient sufficiently difficult to prove. Cru veilhier, who seems to lean towards Majendte's opinion, admits nevertheless several weighty objections to it. The most iinportant of these appears to me to be that the margins of the oritice which is brought into view by the me thod directed by Majendie, are irregular, and have the appearance of lacemted membrane. And it is recorded by M. Martin St. Ange, on
the authority of Cruveithier, 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 bra.nes."* 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 reinoval, or in the manipulations necessary for demonstrating it. It appears to me that the fourth ventricle is closed in the sarne 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 occ,asion 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-amchnoid space.-1 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 I 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 intra-ventrieular 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 I use Majendie's phrase) of the sub-arachnoid fluid.