NERVOUS SYSTEM. (NERVOUS CENTRES. ABNORMAL ANATOM f.) 713 



nion'.y affect the tissues of the same kind oc- 

 curring in other parts of the body. 



Inflammatory affections of the dura mater are 

 exceedingly rare, and occur chiefly in connexion 

 with wounds or injuries of the spine, or in 

 extension of disease from the bones. Occa- 

 sionally but very rarely we find osseous or 

 cartilaginous deposits upon it, which are most 

 obvious on its arachnoid surface. Blood is 

 sometimes, but not frequently, effused externally 

 to it, and effusions of serous fluid are still more 

 rare. Such effusions, from the usually depen- 

 dent position of the spinal canal, and from the 

 large venous plexus which exists around the 

 dura mater, are very likely to be pseudo-morbid, 

 resulting from the gravitation of the fluids after 

 death. Cancerous or fungoid tumours may 

 originate from the dura mater, or may arise 

 externally and grow to it afterwards. Tuber- 

 cles may form between the dura mater and its 

 arachnoid lining. 



When a deficiency of more or less of the 

 posterior osseous wall of the spinal canal occurs, 

 we find a corresponding dilatation of the dura 

 mater and arachnoid sac, which, being filled 

 with water, causes an external tumour, consti- 

 tuting what has been called Hi/drorachis, the 

 consequence of spina bifida. These tumours 

 are altogether dependent on the congenital 

 imperfection of the bones of the spine, and 

 whatever peculiar disposition of the spinal cord 

 or its nerves may be found within them is due 

 to an arrest of or a disturbance in the process of 

 developement of those parts. The details of the 

 anatomy of these tumours will be found in the 

 article SPINE. 



The arachnoid. the spinal arachnoid exhi- 

 bits marks of the inflammatory process more fre- 

 quently than thed lira mater. But in neither mem- 

 brane does this state of disease occur often, ex- 

 cept as a complication of injury or of a morbid 

 state of other parts, either of the vertebral column 

 or of the spinal cord itself. The signs of an 

 inflamed state of this membrane are lymph 

 effused on its free surface, recent, or indurated 

 causing more or less thickening. Adhesions be- 

 tween corresponding parts of the two arach- 

 noid layers are also a good indication of a 

 previously existing inflammation. But care 

 must be taken not to mistake the adhesion, 

 which is often found between points of these 

 membranes, tor inflammatory adhesion. The 

 former occurs in minute points, and is probably 

 a result of drying of the membranes at the 

 points of contact; the latter is always ac- 

 companied by the formation of new matter 

 which forms the medium of union between the 

 layers. 



Cartilaginous spots are by no means unfre- 

 quently found on the arachnoid membrane, 

 chiefly in connection with its visceral layer. They 

 are generally small detached laminae thoroughly 

 incorporated with the membrane, rarely exceeding 

 in size the flat surface of a split pea, more fre- 

 quently much smaller. They generally occur on 

 the posterior surface of the arachnoid. I have 

 seen such deposits in cases where there were no 

 previous symptoms to denote any affection of 

 the central nervous system, and I am disposed 

 VOL. in. 



to believe that such deposits, separated as they 

 must be from the surface of the cord, are not 

 likely to occasion much if any irritation to that 

 organ. They are found mostly in the dorsal and 

 lumbar regions. Sometimes spots of bone, of 

 similar size and shape, are found scattered over 

 the membrane. 



Thepia mater. The spinal pia mater being 

 the seat of the vascular ramifications which con- 

 tribute to the nutrition of the cord, is also sub- 

 ject to congestions often depending on causes 

 quite extraneous to the spinal canal or cord. 

 Thus the congestions which are produced in 

 animals drowned, or asphyxiated in any other 

 way, exist in the vessels of this membrane. The 

 most frequent cause of congestion in these 

 vessels, however, is the position of the corpse 

 after death. After deaths, preceded by violent 

 convulsions, there is always congestion of the 

 vessels of the pia mater. But this congestion 

 must be regarded as a consequence, not as a 

 cause of the convulsions. The holding of the 

 breath, which accompanies continued convul- 

 sions, gives rise to a very general congestion of 

 the venous system. 



When the congestion is very considerable, it 

 may occasion rupture of bloodvessels and 

 effusion of blood into the sub-arachnoid cavity. 

 This constitutes a form of spinal apoplexy, which 

 is apt to follow concussion of the cord, caused 

 by a fall or by a blow inflicted upon the back. 

 It may follow any of those diseases which are 

 accompanied by convulsions tetanus, hydro- 

 phobia, epilepsy, cerebral apoplexy. 



Inflammatory affections of the membranes, 

 deposits of tubercle or other foreign matter 

 which may cause induration of the cord, have 

 their primary seat among the vessels of the pia 

 mater. Inflammation of the membranes is 

 more apt to occur among children than in 

 adults. 



Abnormal anatomy of the spinal cord. The 

 absence of this organ (ami/elia) occurs chiefly 

 in anencephalous foetuses. In such cases the 

 posterior wall of the spinal canal is often defi- 

 cient, and the canal is occupied by a reddish, 

 vascular pulpy substance. It is a question 

 whether the absence of the cord, in such cases, 

 is to be attributed to a real defect of its deve- 

 lopement or to its destruction while yet in a 

 very delicate semi-fluid state, by the formation 

 of a dropsical effusion, either around it or in 

 the canal or ventricle which exists in it at an 

 early period of its developement. This latter 

 explanation is rendered probable by the fact 

 that all the recorded cases are of foetuses which 

 had reached an advanced period of intra-uterine 

 developement; and in some of them move- 

 ments had been distinctly felt by the mothers, 

 which could not have taken place with a com- 

 bined ordefinite character without the existence 

 of the cord. And in some of the records it is 

 affirmed that the children lived some hours and 

 exhibited movements and even signs of sensa- 

 tion, or at least of excitability to stimuli. 

 Such phenomena, if true, leave us no alterna- 

 tive but to suppose that the whole cord could 

 not have been absent some portion must have 

 existed as the centre of these movements, 



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