NERVOUS CENTRES. (HUMAN ANATOMY. THE MENINGES.) 



633 



cavernous sinuses of opposite sides across the 

 basilar process of the occipital bone. Some- 

 times there are two running parallel to each 

 other. Ouveilhier states that the capacity of 

 this sinus is much greater in old than in young 

 subjects. 



Cavernous sinuses. In point of shape these 

 sinuses differ considerably from all the other 

 sinuses of the dura mater. They are venous 

 reservoirs, situated on each side of the sella 

 Turcica, from which they are separated by the 

 internal carotid arteries. Their name is derived 

 from the spongy appearance which they present 

 in their interior, owing to the existence of some 

 filaments within them, which, by their inter- 

 lacement with each other, form a reticular 

 texture there. It was formerly supposed that 

 the carotid arteries lay in the cavity of these 

 sinuses and were bathed by their blood ; but it 

 is easy to demonstrate by a little careful dis- 

 section that the inner membrane of the sinus 

 adheres loosely to the outer wall of the artery, 

 and that the sixth nerve passes between them. 

 In the outer wall of each cavernous sinus there 

 are channels for the reception of those nerves, 

 which pass from the cranium into the orbit. 

 These are the third nerve which is placed 

 highest up, the fourth nerve which holds the 

 next place, and the ophthalmic portion of the 

 fifth. The cavernous sinus receives at its ante- 

 rior extremity the ophthalmic vein, which col- 

 lects the blood from the eye-ball and other 

 structures within the orbit, and which commu- 

 nicates also with the angular vein and with the 

 frontal vein. (Hence the injected state of the 

 vessels of the eye-ball when the brain is con- 

 gested, as in fever.) Veins from the inferior 

 surface of the anterior lobes of the brain also 

 open into it, also some from the middle lobe 

 and from the dura mater. Posteriorly it com- 

 municates with both the petrosal sinuses, and 

 veins from the cranial bones open into its 

 superior wall. 



Circular sinus. A communication is esta- 

 blished between the cavernous sinuses by means 

 of the circular or coronary sinus which em- 

 braces the pituitary body, one portion lying in 

 front of it and the other behind it, both open- 

 ing by a common free orifice into the right and 

 left cavernous sinuses. The posterior portion 

 of the circular sinus is much larger than its 

 anterior portion. Its size is much greater, 

 according to Cruveilhier, in old subjects than 

 in young ones. It receives small veins from 

 the pituitary body, and also from the sphenoid 

 bone and from the dura mater. 



It is impossible to examine this complicated 

 arrangement of venous channels in connexion 

 with the dura mater of the brain without ad- 

 miring the beautiful provision which it affords 

 against the undue accumulation of blood in 

 the venous system within the cranium. In 

 the first place, we observe that these veins do 

 not admit of dilatation beyond a prescribed 

 extent, by reason of their being enclosed be- 

 tween layers of an inelastic and inextensible 

 membrane. Next, we remark the safety pro- 

 vision which is afforded by the frequent com- 

 munication between them, so that if one chan- 



nel were altogether closed or materially con- 

 tracted, there are many others by which the 

 blood could return. Nor is a local congestion 

 likely to take place to any extent, for such 

 is the freedom of communication between the 

 sinuses and the veins of the exterior of the 

 cranium, that (all being devoid of valves) an 

 overflow would readily be received by the 

 latter without the least impediment. Lastly, 

 we learn the great importance and value of 

 local depletion as an agent for relieving vascu- 

 lar fullness within the head, owing to the free 

 communication between the extra- and the 

 intra-cranial circulation, and especially of the 

 veins ; and we may infer from anatomy that 

 local depletion would most probably be more 

 serviceable than general, for although the latter 

 would diminish the amount of the mass of 

 circulating fluid, it would not affect the relation 

 between the venous and arterial systems, whilst 

 it is evident that the former must affect the 

 venous system more directly than the arterial. 

 Moreover, the free communication between the 

 circulation within and that without the cranium 

 may explain somewhat the advantage that is 

 often derived from the application of an intense 

 cold to the external surface of the head. 



Of the pia muter. ( Tunica intiina vel vascu- 

 losu.) The pia mater is the most internal mem- 

 brane of those which have been enumerated 

 as belonging to the spinal cord and brain. 



Pia mater of the spinal cord. This mem- 

 brane stands in precisely the same relation 

 to the spinal cord as the neurilemma does to 

 the nerves ; and as long as the spinal cord 

 could be, as it formerly was, regarded merely 

 as a bundle of nervous fibres, the analogy of 

 this membrane to the nervous sheath would 

 be perfect. It is composed almost entirely 

 of white fibrous tissue; it closely invests the 

 cord and supports the minute bloodvessels 

 which carry the nutrient fluid to it. Not 

 only does it thus form a complete sheath to the 

 cord, but it likewise sends in processes which 

 dip into the anterior and posterior median 

 fissures of that organ. That which passes into 

 the anterior median fissure is a true fold or 

 duplicature of the pia mater; but the posterior 

 fissure, which is much narrower than the ante- 

 rior, is occupied only by a single and extremely 

 delicate layer, which at some parts almost 

 entirely disappears, and seems to consist merely 

 of a few minute capillary vessels. The pia mater 

 becomes continuous with the neurilemma of 

 the roots of the nerves on each side of the cord, 

 and at its inferior extremity it tapers in accor- 

 dance with the shape of the spinal cord, and is 

 prolonged as a delicate thread which is inserted 

 into the extremity of the dura mater. This 

 prolongation is quite gradual, so that at the 

 upper part it encloses a portion of the medullary 

 substance of the cord ; in the greater part of its 

 extent, however, it is merely a membranous 

 thread, and, therefore, goes by the name fili- 

 form prolongation of the pia mater '(jilum 

 terminate). The late Dr. Macartney used 

 to regard it as highly elastic, but my friend 

 Mr. Bowman has called my attention to the 

 fact that it consists almost entirely of white 



