574 MEMBRANES OE BRAIN AND CORD. 



existence of a sort of septum {sejihim posticum), dividing the subarach- 

 noid s]mce at the back of the cord, the relations of ^vhich have been 

 carefully studied by Axel Key and Eetzius. It is a thin membranous 

 partition, -which passes in the median plane from the pia mater covering 



Fig. 395. — Transverse Sectiox of the Spixal 

 OoRD AND ITS ENVELOPES (froiu Sappej after 

 Hirschfeld and Leveille). 



1, dura mater or tlicca ; 2, supposed parietal 

 layer of tlie arachnoid membrane ; 3, internal or 

 loose arachnoid ; 4 and 7, subarachnoid cavity or 

 space ; 5, hinder part of the an tero- lateral 

 column ; 6, subdural space between the arach- 

 noid and the dura mater ; 8, supposed reflection 

 of the one fold of the arachnoid into the other ; 

 i>, sheath furnished to the spinal nerve by the 

 dura mater ; 10, posterior ganglionic root ; 11, smaller anterior root ; 12, section of the 

 ligamentum denticulatum. Tliis tigure does not show tlie septum which posteriorly 

 divides the subarachnoid space into right and left parts : this ^\•ould be placed between 

 the arachnoid at 3, and the pia mater covering tlie posterior surface of the cord. 



the posterior median fissure of the cord to the opposite part of the loose 

 portion of the arachnoid membrane. It is most perfect in the cervical 

 region, being incomi)lete below, and consists of numerous fine lamellse, 

 enclosing between them small spaces, within which run the larger 

 blood-vessels. Trabecular connect the nerve-roots with the inner surface 

 of the arachnoid, and in the dorsal region fine membranes extend be- 

 tween the posterior nerve-roots and the posterior septum. 



The nerves as they pass from the brain and spinal cord receive two 

 sheaths, an outer from the dura mater, and an inner from the arachnoid. 

 Upon the optic nerve these sheaths remain distinct and separate, so that 

 the space which each encloses may be injected, the outer from the sub- 

 dural, the inner from the subarachnoid space. On the other nerves the 

 arachnoidal sheath soon ceases, and the single sheath maybe injected from 

 either the subdural or subarachnoid cavity. Separate sheaths surround 

 each bundle of the larger nerves, and the injection passes readily along 

 the nerves even as far as the limbs. 



There thus exists a continuity between the ventricles of the brain, 

 the subarachnoid space, the perivascular canals of the cerebral sub- 

 stance, and the perineural spaces within the nerve sheaths. 



Structure. — When examined under the microscope, the arachnoid 

 is found to consist of distinct riband-like bundles of fine fibrous tissue 

 interlaced with one another. The intervals between these bundles are 

 filled up by delicate membranes, composed of expanded cells, the nuclei 

 of which persist and are scattered over the structure. Several layers of 

 this tissue, arranged in a complex way, constitute the arachnoid mem- 

 brane. The subarachnoid trabeculte consist of bundles of similar fine 

 fibrillar tissue, each of which is surrounded by a delicate nucleated 

 sheath, also composed of cells, and continuous with the inter trabecular 

 cell-membranes of the arachnoid itself. Some of the finer trabeculEe 

 are said to be surrounded by a spiral fibre. The subarachnoid mem- 

 branous expansions have a similar structure. \'olkmann has described 

 a rich plexus of nerves in the arachnoid membrane of certain ruminants. 

 Kolliker has failed to detect their presence; but they have been again 

 described by Bochdalek, who traces them to the portio minor of the 



