COVERINGS OP THE NERVOUS CENTRES. COVERINGS OF THE GANGLIONS. — Every ganglion is covered by a more or IA. dense layer of white fibrous tissue, similar to that which forms the neurilemma of nerves. It per forms precisely the same office for the elements of the ganglions that the neurilemma does for those of nerves; that is, it gives them a me chanical support, and is the medium through which bloodvessels are conveyed to their ner vous matter. It is continuous with the neu rilernma of the nerves which are connected with the ganglions. It is found in all forms and classes of ganglions, presenting the same essen tial characters. These bodies are generally surrounded by and imbedded in a considerable quantity of fat, which also involves more or less the nerves that proceed from them.
CovEarises OF THESPINAL CORD AND BRAIN. —These are also called the membranes of these centres, or the menirw-es membrana). They are three in number. Those of the brain are continuous with those of the spinal cord, but, as there are certain distinctive characters proper to each, it will be convenient to describe the cerebral and spinal rneninges separately. They are, enumerating them from without in wards, the dura tinter, the arachnoid mem brane, and the pia mater.
The term, mater, ILVT7je originated with the Arabian anatomists, who regarded these mem branes as the parents of all others in the body. Galen adopted the word tolny!, and distin guished the first and last of the membranes above enumerated by the adjectives vraxvvvo and Avvrrn. The Germans use the word heart, and de.signate these membranes as hautige Hullers des Gehirns rind des Ruckenmarkes ; die harte Hirnhaut, die hiirte Ruckenmark haut, the dura mater of the brain and spinal cord; die Spinnwebenhaut, the arachnoid; and die weiche Haut, the pia mater.
Dura mater.—The dura mater is a dense membrane composed almost exclusively of white fibrous tissue. It has all the characters, physical and vital, of that texture, possessing great strength and flexibility with but little elasticity. It is freely supplied by blood vessels, and at certain situations, which will be more particularly described by-and-bye, it separates into two laminm, which inclose prolongations of the lining membrane of the venous system, forming peculiar sanguiferous channels, which are commonly known by the name of sinwes. It has an apparent lamellar
disposition, from the fact of its fibres being arranged in different planes. In the child a subdivision into two layers may sometimes be easily effected. Some nerves have been de monstrated in the dura mater; a branch of the fifth nerve has been particularly described and delineated by Arnold, a.s passing in a recur rent course between the laminve of the tento rium, and Pappenheim has found nervous fibres in the cerebral dura mater derived from the superior maxillary division of the fifth, from the fourth nerve, from the vidian, and probably also from the frontal branch of the ophthalmic..
The spinal dura ranter 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. It adheres 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 laminw 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.