y///: CBRSBBO-SPntAL AM.-* coi 



nsol\v> it-, it into two layers: au external, applied to the inner face of the 

 duni mater ; and an internal, spread, through the medium of the pia mater. 

 over the cercbro-spinal axis, from which it is again separated in a great 

 number of points by a particular fluid, the subarachnniil. 



The I'i't iinif' i\ or iniirital meninge, is the proper envelope of tho central 

 in TVOUS stalk ; it is cellule- vascular, closely adherent to the external surface 

 of the cord, united to the visceral layer of the arachnoid by more or less 

 dense connective tissue, between the meshes of which is deposited the 

 subarachnoid fluid. 



This arrangement of the cerebro-spinal envelopes permits the cerebro- 

 spinul axis to be assimilated, to a certain extent, to a viscus, and the bony 

 sb: aib containing them to a splanchnic cavity, whose serous membrane, tho 

 u mi-Inn 'ill, is covered outside its parietal layer by a fibrous expansion, the 

 ihn-ii muter, and within its visceral layer by a cellulo-vascular tunic, the 

 pia mater, or inf< rnl mminge. 



This collective view of the envelopes belonging to the nervous centres 

 will now be followed by a special description of each, in which their spinal 

 and cranial portions will be successively considered, after glancing at them 

 in a general manner. 



1. The Dura Mater. 



This membrane is the most external aivd the strongest of the cerebro 

 spinal envelopes, and covers the walls of the cerebro-spinal cavity, whose 

 form it exactly repeats. It is, therefore, a second protective sheath, which 

 is dilated at its anterior extremity into an ovoid cavity that lodges tho 

 encc phab>n, and terminates in a prolonged point in the coccygeal vcrtebrne. 

 It offers two faces : an external, in contact with the walls of the bony 

 case ; and an internal, adhering in the most intimate manner to the external 

 layer of the arachnoid. 



In several points of its extent it is traversed by the nerves which escape 

 from the cerebro-spinal axis, and by the vessels destined to this portion of 

 the nervous system. 



STUCCTURE. The dura mater possesses the texture of all white fibrous 



membranes. It is composed of parallel longitudinal fasciculi of connective 



tissue, mixed with some fine elastic fibres. Bourgelat thought they formed 



istinct layers au external and internal; but nowhere is it possible to 



nstrate this. It receive s Llmxl-vessels ; the arteries are dcrivid. for the 



spinal portion, from the vertebral, tho intcrcostals, lumbar, and lateral 



sacral s ; for the cranial portion, mcningcal minuscules, such as the ethmoidal 



branch of the nasal, the spheiio-spinoiis, and tympanic, mastoideal, and 



ro-spinal arteries. Nerves have been M- n passing to its cranial portion ; 



have been divided into anterior, middle, and posterior. Tho first 



: irnishcd by the ethmoidal filament of the nasal nerve ; tl;e s. i..i.d from 



.-Jan ganglion; and the third, by the ophthalmic branch of "Willis. 



of /////(;</.'//< iv </.< has m.t \vt \> en demo: 



Si'iN.M. Drr.A MATKK (TiiKc.v VKIITEJIKAMS). This is a v ( ry elongated 



sheath. < ntinuoUK at the occipital foramen with the encephalic dura mt<T. and 



! b< hind by an attenuated point h.dged in the narrow channel which, 



in the middle coccygeal verte! i :.ts a tra-c of the spinal cai.al. 



is in si. aj xuctly like the luttx r. its largest diameter is i:t the 

 and at the brachial and lumbo -. . j.al c.-rd. Its 



iepends greatly on the volume of tl. ;.nd in some . 



it can allow the accumulation of the ccrebro~pinal fluid : 



