NEUROLOGY. 107 



usually bound up together in the same sheath. The precise relations of 

 these two sets of fibrous nerves to each other and to the nervous centres 

 will be subsequently referred to. 



We shall now proceed to consider the special anatomy of the nervous 

 system, under the heads of the brain, the spinal cord, the nerves, and the 

 ganglions. 



2. Anatomy of the Brain. 



Ä. Membranes of the Brain and Spinal Cord. 



The brain is that ganglionic enlargement of the cerebro-spinal axis seen 

 at its anterior extremity, and characterizing the nervous system of the ver- 

 tebrata, as distinguished from the invertcbrata. Deferring for the present 

 any further account of its intimate structure, we find it to be invested with 

 three membranes or meninges : one, fibrous and external, the dura mater ; 

 one, serous and median, the arachnoid ; and a third, vascular and internal, 

 the pia mater. 



1. Dura Mater. The dura mater is a dense membrane, composed 

 almost entirely 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 with blood-vessels, and at certain 

 situations it separates into two lamime, which inclose prolongations of the 

 lining membrane of the venous system, forming peculiar sanguiferous chan- 

 nels, known as sinuses (see p. 97). It has an apparently laminated appear- 

 ance, from the fact of its fibres being disposed in different planes ; indeed, 

 in the child a subdivision into two layers may sometimes be easily effected. 

 Some nerves have been demonstrated in the dura mater. 



The spinal dura mater is in shape adapted to the vertebral canal, consist- 

 ing of a hollow cylinder, tapering somewhat at its lower extremity to cor- 

 respond with the sacral portion of the canal. It adheres very firmly around 

 the foramen magnum, from which it is continued downwards to the sacrum, 

 without forming any adhesion to bone. 



On the sides the dura mater is perforated 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 anterior root of each nerve, is separated from that of the pos- 

 terior root by a narrow strip of fibrous membrane. Although not attached 

 to any part of the vertebral canal between the foramen magnum and the 

 sacrum, 3^et the spinal dura mater is sustained in its position by the prolon- 

 gations of its substance over the nerves at each of the intervertebral 

 foramina. It is also decidedly larger than would be necessary merely for 

 the reception of the spinal cord ; as, when pierced, a quantity of fluid 

 escapes, and it becomes quite flaccid, to be again rendered tense by inject- 

 ing water or air. 



Cranial dura mater. The dura mater of the cranium differs in one 



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