716 



NERVOUS SYSTEM. (NERVOUS CENTRES. ABNORMAL ANATOMY.) 



W hen there has been a solution of continuity 

 and a loss or removal of any portion of the 

 cranium, the exposed surface of the dura mater 

 is apt to throw out a growth of granulations 

 which constitute the fungus of the dura mate?; 

 analogous to that which sprouts from the sur- 

 face of a similar fibrous membrane the tunica 

 albuginea of the testicle. In point of structure 

 this fungoid growth is the same as the granula- 

 tions on the surface of external ulcers. 



Effusion of blood, constituting a form of 

 meningeal apoplexy, may occur on the external 

 surface of the dura mater separating it from the 

 bone; or on its internal surface, dissecting 

 away the arachnoid membrane from its adhesion 

 to the dura mater. The former kind is mostly 

 if not always traumatic, that is, resulting from 

 the application of violence to the exterior of the 

 cranium The latter kind is of extremely rare 

 occurrence, and must be carefully distinguished 

 from that variety of effusion into the arachnoid 

 sac in which the effused blood appears to be 

 covered by a serous membrane. This mem- 

 brane, however, results from the condensation 

 of the superficies of the clot by its friction 

 against the parietal arachnoid, and it may be 

 distinguished from a true serous membrane by 

 the absence of epithelium from its free surface. 

 Effusions of either kind generally occur on 

 some part of the surface of the cerebral hemi- 

 spheres above the level of the petrous bone. 



The arachnoid membrane. The arach- 

 noid membrane is sometimes the seat of 

 acute inflammation, and presents the same 

 signs of that process as are met with in other 

 serous membranes. The chief and, indeed, 

 the only unequivocal sign of his condition 

 as of recent occurrence is the exudation of 

 plastic lymph upon the free surface of either or 

 both layers of the membrane, with or without 

 pus. This is attended with a highly injected 

 state of the subjacent tissue (pia mater or dura 

 mater, generally the former). The arachnoid 

 itself, it will be remembeied, contains no blood- 

 vessels, but derives its nourishment from the 

 vessels of the subserous tissue. Its apparent 

 vascularity is due to its great tenuity and trans- 

 parency, which allow the bloodvessels lying 

 underneath to be seen through it as if they be- 

 longed to the membrane itself. 



An opaque condition of the arachnoid, vary- 

 ing both in degree and extent, is a very common 

 appearance of this membrane, especially at the 

 middle and advanced periods of life. This 

 occurs sometimes in patches; at other times it 

 is generally diffused over the whole membrane. 

 It is most conspicuous on the convex surface 

 of the brain, especially towards the great longi- 

 tudinal fissure, and it is frequently associated 

 with large and numerous Pacchionian bodies. 

 It occurs, however, very commonly at the base, 

 and frequently opposite the confluxes of the 

 subarachnoid fluid. 



The opacity of the arachnoid is commonly 

 attributed to a former acute inflammation of 

 the membrane, or to a chronic inflammation 

 going on up to the time of death. But this 

 state of the membrane is of such frequent oc- 

 currence, and is so often found in persons who 



evinced no sign of important organic change 

 during life, that it seems scarcely correct to 

 attribute it to such a cause. It is not meant 

 to deny that previous inflammation or chronic 

 inflammation is capable of causing these 

 opaque spots, but undoubtedly other causes 

 may produce them as well. The friction of 

 two opposed surfaces may do it, and deposits 

 upon the free surface of the membrane, an al- 

 tered condition of the epithelium, may have 

 the same effect. Some recent microscopical 

 examinations convince me that morbid deposits 

 similar to those which are formed on the coats 

 of arteries, may be found here, and occur in 

 those morbid states of the blood, and conse- 

 quently of the whole system, which are favour- 

 able to the deposition of a morbid material 

 throughout the arterial system, or in the sub- 

 stance of viscera.* 



In confirmation of this view it may be stated 

 that opacities of the arachnoid are most com- 

 mon after the middle period of life, and that 

 they are then almost uniformly associated with 

 a morbid state of the arteries of the brain and 

 of other portions of the arterial system. 



Adhesion between the opposed surface of 

 the visceral and parietal layers of the arachnoid 

 is (and the fact is curious) not of frequent oc- 

 currence, excepting at the convex border of the 

 falx cerebri, where the Pacchionian bodies are 

 found. And the intrusion of these bodies into 

 the longitudinal sinus frequently increases the 

 closeness of that adhesion. The cellular ad- 

 hesion so common in other serous membranes 

 is rarely found in the arachnoid. 



Small plates of cartilage or of bone are some- 

 times found in connection with the arachnoid. 

 Their formation is generally the result of a pre- 

 vious morbid deposit which has subsequently 

 become converted into cartilage or bone. 



Effusion. Effusions take place either into 

 the subarachnoid or the arachnoid cavity. The 

 existence of serum, in undue quantity, in the 

 former situation, must be looked on as an in- 

 crease in the fluid which naturally occupies 

 that space, and as we have already remarked 

 in a former part of this article, it takes place in 

 consequence of the failure of the normal pres- 

 sure upon the vascular surface, and may rather 

 be regarded as tending to preserve the functions 

 of the brain than as producing an injurious 

 pressure upon it. Indeed I have always found 

 that in cases where an abnormal quantity of 

 fluid existed in the subarachnoid cavity, the 

 brain afforded no indication of its having expe- 

 rienced undue pressure previous to death. In 

 such cases the brain seems to contain less 

 blood than natural, and its anannia is most 

 obvious in the grey matter. Sometimes there 

 is hyperaemia of the veins in the white matter 

 of the hemispheres, as if the heart's force, more 

 necessary to the venous circulation within the 

 cranium than even to that of other parts of the 

 body, had been prevented from exerting its 

 influence through the capillaries upon the 

 blood in the veins. 



* It is probably to deposits of this kind that Ro- 

 kitansky refers under the title of " Gallcntettige 

 Concretioncn." 



