636 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE MENINGES.) 



sabulous matter consists chiefly of phos- 

 phate of lime with a small proportion of phos- 

 phate of magnesia, a trace of carbonate of lime, 

 and a small quantity of animal matter. 



The pia mater adheres very closely to the 

 surface of the brain, coming for the most part 

 into contact with grey matter. When a portion 

 of it is raised carefully in a fresh brain, num- 

 berless extremely minute bloodvessels are seen 

 passing from it into the cerebral substance. 

 These are the principal nutrient vessels of the 

 brain. On its outside the pia mater adheres 

 partially to the arachnoid membrane. At those 

 points which correspond to the convex portions 

 of the convolutions the adhesion of arachnoid 

 to pia mater is close ; but at other places the 

 latter membrane separates completely from the 

 former. 



The pia mater of the brain differs from that 

 of the spinal cord in its great delicacy and 

 tenuity ; it wants the strength and density of the 

 latter membrane. This is owing to its being 

 composed almost entirely of extremely minute 

 and delicate bloodvessels, whilst the spinal 

 membrane consists chiefly of white fibrous 

 tissue. The bloodvessels of the former are in- 

 finitely more numerous than those of the latter, 

 and the reason of this probably is that the 

 cerebral membrane is chiefly in contact with 

 grey matter, which requires a great quantity of 

 blood, but the spinal membrane immediately 

 embraces white matter, which is much less 

 vascular. 



It is important, in a pathological point of 

 view, to notice that this membrane is the me- 

 dium of nutrition, not merely to the nervous 

 matter of the brain and cord, but also to the 

 arachnoid membrane which is immediately 

 adherent to it, and to which it bears the same 

 relative position as the sub-serous areolar tissues 

 elsewhere to their respective serous membranes. 

 Hence the difficulty, if not the impossibility, 

 of adopting distinctions which systematic wri- 

 ters endeavour to make out between arachnitis 

 and superficial inflammation of the brain. It 

 is physically impossible that there shall be 

 arachnitis without serious disturbance of an 

 inflammatory kind in the circulation of the 

 pia mater, nor can this exist without affecting 

 the superficial layers of the grey matter of the 

 convolutions. It may, therefore, be confidently 

 affirmed that arachnitis, when affecting that 

 portion of the arachnoid membrane which co- 

 vers the hemispheres of the brain, is synony- 

 mous with inflammation of the superficial 

 layers of the grey matter of the convolutions. 

 Whatever be the point of departure, it seems 

 impossible that inflammation of the one can 

 exist without a similar and equal affection of 

 the other. And thus we may explain the ap- 

 parently anomalous statement of authors that 

 inflammation of the arachnoid should give rise 

 to a more violent train of symptoms than deep- 

 seated inflammation of the brain. The real 

 difference is, not between membranous and 



Bcremann, liber die innern Organisation des 

 Gehiins. The last author stales that he has seen 

 the sandy deposit excessive in connexion with 

 mental derangement. 



cerebral inflammation, but between an inflam- 

 matory affection of the superficial grey matter 

 of the convolutions, the great source and seat 

 of the physiological activity of the brain, and 

 a similar morbid action of the more central 

 white substance, the function of which is in a 

 certain sense subservient to that of the super- 

 ficial grey matter. 



Of the arachnoid membrane. This mem- 

 brane is intermediate to those already described. 

 We have preferred giving the description of it 

 last, because to understand it demands an 

 acquaintance with the details of both those 

 membranes. 



The arachnoid is a great serous membrane 

 pervading the entire cranio-spinal cavity. Its 

 parietal layer adheres intimately and insepara- 

 bly to the inner surface of the dura mater both 

 cranial and spinal, and its visceral layer is 

 attached to the outer surface of the pia mater. 



In point of structure and general disposition 

 the arachnoid membrane resembles other se- 

 rous membranes, so much as to render it inex- 

 pedient to enter into any minute comparison 

 of them. It will only be necessary to refer to 

 such peculiarities of arrangement as may arise 

 from the anatomical characters of the nervous 

 centres with which it is connected. 



Spinal arachnoid. The serous character of 

 the spinal arachnoid is best seen by examining 

 a transverse section of the spinal cord and its 

 membranes. If the section be made across 

 the interval between two sets of spinal nerves, 

 the visceral and parietal layers of the mem- 

 brane may be seen in contact with each other ; 

 the parietal layer closely attached to the dura 

 mater, the visceral layer adherent to the pia 

 mater of the spinal cord so loosely as to leave 

 a considerable space between it and the outer 

 surface of that membrane. 



Fig. 366. 



Transverse section of spinal cord and its membranes 



between the fifth and sixth cervical nerves. 



(After Arnold.) 



v, visceral layer of arachnoid membrane ; g, sub- 

 arachnoid space ; c, arachnoid cavity. 



We may here notice an important distinction 

 which the student of this portion of anatomy 

 will do well to note particularly, namely, that 

 the space between the two layers of arachnoid 

 membrane is the arachnoid bug or sac, in 

 which it is very rare for any fluid to accumu- 

 late; and that that between the visceral layer of 

 the arachnoid and the pia mater is the sub- 

 aruclmoid cavi'.y, in which, as will be shown 

 by-and-bye, a considerable quantity of fluid 

 exists in the natural state. 



When the section is made on a level with the 

 nerves as they emerge through the dura mater, 

 we may notice the manner in which the arach- 

 noid membrane is prolonged upon the nerves 

 in the form of a loose sheath, forming little 



