628 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE MENINGES.) 



The spinal dura mater is in shape adapted 

 to the vertebral canal. It is a hollow cylinder, 

 tapering somewhat at its lower extremity to cor- 

 respond with the sacral portion of the canal. 

 Itadheres very firmly all round the foramen mag- 

 num of the occipital bone. From thence it is 

 continued down to the sacrum without forming 

 any adhesion to bone. On the posterior and 

 lateral surfaces it is covered by a layer of soft, 

 oily, reddish fat, which intervenes between it 

 and the inner surfaces of the vertebral laminae 

 and processes, and in these situations, as well 

 as to a less degree in front, we find a very in- 

 tricate plexus of veins, some of which are of 

 considerable size. The fatty deposit is most 

 abundant in the sacral region. In front the dura 

 mater adheres by a close areolar tissue to the 

 posterior common ligament, and here of course 

 the adipose tissue is deficient. At the foramen 

 magnum the continuity of the spinal dura 

 mater with that of the cranium is distinct, and 

 here, indeed, the former appears as a funnel- 

 shaped prolongation of the latter; both are, 

 in truth, portions of the same membrane 

 adapted to the difference of shape of the ner- 

 vous centres with which they are respectively 

 connected. 



On the sides the spinal dura mater is per- 

 forated by orifices which give exit to the roots 

 of the nerves which arise from the spinal cord. 

 When examined from within, these foramina 

 are found to be arranged in pairs, each pair 

 corresponding to the point of exit of a spinal 

 nerve. The foramen which transmits the an- 

 terior root is separated from that which gives 

 passage to the posterior one, by a narrow slip 

 of fibrous membrane. These foramina are 

 slit-like in form, taking the vertical direction. 

 On the outer surface of the dura mater the 

 distinction between them is not evident with- 

 out dissection, for there the fibrous membrane 

 being prolonged from the margins of the open- 

 ings around the nerves, the sheaths thus formed 

 coalesce and surround both roots. The number 

 of these orifices is of course the same as that 

 of the roots of the nerves which pass through 

 the dura mater. 



The internal surface of the spinal dura mater 

 is perfectly smooth and moist in the healthy 

 stale, owing to its being lined by the parietal 

 layer of the arachnoid membrane. In the 

 intervals between the orifices for the transmis- 

 sion of each pair of spinal nerves, it receives the 

 pointed attachments of the ligamentum denti- 

 culatum, to be described more fully by-and- 

 bye. 



It is evident from the preceding description 

 that the spinal dura mater cannot perform the 

 office of a periosteum to the osseous walls of the 

 spinal canal, for at every point it is separated 

 from them by texture of a different kind, and, 

 moreover, the vertebrae are provided with a 

 distinct periosteum. The prolongations of dura 

 mater over the nerves at each of the interverte- 

 bral foramina serve to fix that membrane at 

 the sides throughout the whole extent of the 

 vertebral canal, so as to prevent its lateral dis- 

 placement. At the lower extremity of the 

 sacrai canal the dura mater ends in a blunt 



point, and from this certain processes may be 

 traced towards the coccyx. Of these the cen- 

 tral one is continuous with the filiform pro- 

 longation from the pia mater, which is inserted 

 into the inferior extremity of the dura mater, 

 and is implanted into the last bone of the 

 coccyx. The thread-like processes which are 

 seen on each side are the sheaths of the last 

 sacral nerves and of the coccygeal nerve, which 

 pass some distance in the canal before they 

 reach the foramina for their transmission out- 

 wards. 



It is easy to convince oneself that the spinal 

 dura mater is far larger than would be neces- 

 sary for the reception of the cord. When the 

 fluid immediately surrounding this organ has 

 been suffered to escape, the dura mater appears 

 quite loose, flaccid, and wrinkled. By blowing 

 air or injecting water into its canal, it may be 

 rendered tense again. This looseness of the 

 dura mater is most conspicuous at its lowest 

 part, in the lumbar and sacral regions, where 

 it forms, as Cruveilhier says, " autour de la 

 queue de cheval une vaste ampoule, qui parait 

 n'avoir d'autre utilite que de servir de reservoir 

 au liquide cephalo-rachidienne." 



The dura mater adapts itself, in point of size, 

 to the varying dimensions of the spinal canal 

 in its different regions, which again appear to 

 be influenced by variations in the dimensions 

 of the spinal cord. Thus, it swells in the 

 cervical and in the lumbar region, at both 

 which places there are corresponding enlarge- 

 ments of the cord. Its most contracted portion 

 is that which occupies the dorsal region. 



Cranial dura mater. The dura mater of the 

 cranium differs in one leading circumstance 

 from that of the spine, namely, that it forms 

 a periosteum to the inner surface of the cranial 

 bones. We find it, therefore, very closely 

 adherent to the whole interior of the cranium, 

 and the free communication between the vessels 

 of the dura rnater and those of the bones serves 

 materially to enhance the connexion between 

 this membrane and the osseous surface. At 

 some situations the adhesion is so very intimate 

 that we experience great difficulty in attempting 

 to separate the fibrous membrane from the sub- 

 jacent bone. On the roofs of the orbits, the 

 wings of the sphenoid bone, the petrous por- 

 tions of the temporal bones, the margin of the 

 occipital foramen, and opposite the sutures, 

 the adhpsion is very intimate. 



This adhesion of the dura mater to the bones 

 is found also to vary in degree at different pe- 

 riods of life. It is very intimate in old age, 

 so much so that, in removing the calvaria, 

 layers of bone often chip off, remaining in con- 

 nexion with the fibrous membrane. In the 

 adult, such a degree of adhesion as would give 

 rise to this effect, ought to be regarded as mor- 

 bid. In the young subject, while ossification 

 and growth are going on, the adhesion is very 

 intimate, so that in them great difficulty is 

 experienced in removing the calvaria. Doubt- 

 less this intimate adhesion at this early period 

 of life is due to the active share which the 

 dura mater takes in conveying the material of 

 nutrition and growth to the cranial parietes. 



