Membranes of the Brain arid Spinal Cord. 



'Sulc. longit. anf. 

 Pia mater 



95 



Pars spinalis 

 durae matris 



Arachnoidea 

 spinalis 



N. SPINALIS 



Pl&c. venos. 

 spinal. 



435. Diagram of the Membranes of the Spinal Cord. 



Transverse Section. Magn. = 2. 



The Dura mater of the cord, Pars spinalis durae matris, con- 

 tinuous with that which invests the brain, extends, below, as far as the 

 top of the sacrum ; beyond this point it is impervious, being continued, 

 in the form of a slender cord, to the back of the coccyx, where it blends 

 with the periosteum. It is not adherent to the bones of the spinal canal, 

 which have an independent periosteum, and the intervening space is occupied 

 by a plexus of veins, Plexus venosi spiiiales. The Dura mater of the cord 

 accompanies each of the spinal nerves into its vertebral foramen, and 

 sends off from its inner surface to each side of the cord, from 20 23 slen- 

 der processes in the form of denticuli, which serve to support the cord, 

 and are collectively called the Liyamentum denticulatum. 



The Arachnoidea is a delicate membrane, situated between the 

 dura mater externally and pia mater internally. The space between 

 the dura mater and arachnoid is called subdural space or Cavum 

 arachnoideale ; that between the arachnoid and pia mater, sub -arach- 

 noid space, Cavum subarachnoideale ; the latter contains a considerable 

 quantity of fluid, the cerebro-spinal fluid, but the former also 

 contains some fluid. The arachnoid passes over the various eminences 

 and depressions on the cerebrum and cerebellum, without dipping down 

 into the sulci and smaller grooves. It invests the brain (Arachnoidea 

 cerebral-is) and spinal cord (Arachnoidea spinalis) much less closely 

 than the pia mater. From the posterior median fissure, Sulcus longitudinalis 

 posterior, of the cord, a septum passes to the inner surface of the Arach- 

 noidea, which is only complete in the cervical region. Upon the external sur- 

 face of the dura mater, in the vicinity of the longitudinal sinus, the Granula 

 (wrongly Glandulae) Pacchioni lie; they are also found in other situations. 



The Pia mater, the innermost membrane, dips down into all the sulci 

 of the brain and into the fissures of the cord, and is intimately adherent 

 to their surfaces. It is a highly vascular membrane ; at the transverse fissure 

 of the cerebrum it is invaginated into the lateral ventricles and over the 

 third ventricle, and there forms the Velum interpositum or Tela choroidea 

 superior ; it is also prolonged over the posterior wall of the fourth ventricle, 

 where it forms the Tela choroidea inferior and choroid plexuses of that 

 ventricle. At the point where the cord terminates, the Pia mater is con- 

 tinued down as a long, slender filament, Filum terminale. 



