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 mound the nerv, 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 state, 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.
lt 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 vertebrx are provided with a distinct periosteum. The prolongations of dura mater over the nerves at each of the intervene bral foratnina 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 sacral 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 flom the pia matey, which is inserted into the inferior extremity of the dura water, 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 coecygeal 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 cold. When the
fluid immediately surrotinding 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 dum 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 Ara 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 mater and those of the bones serves materially to enhance the connexion between this membrane and the osseous surfitce. 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 spbenoid bone, the petrous por tions of the temporal bones, the margin of the occipital foramen, and opposite the sutures, the adhesion 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. 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 penod 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.