I3IO 



A MANUAL OF ANATOMY 



dorsal compartment. The dorsal compartment is partially sub- 

 divided into two portions, right and left, by means of a third in- 

 complete septum, called the septum posticum. This partition 

 extends from the pia mater, as it crosses the dorsal median fissure 

 of the spinal cord, to the dorsal part of the arachnoid at the median 

 line. All the compartments of the subarachnoid space communicate 

 freely with each other. 



The subarachnoid space has no communication with the subdural 

 space. 



Structure of the Arachnoid Membrane. — ^The arachnoid consists of fine 

 fibrous tissue, arranged in interlacing bundles, the intervals between these 

 bundles being occupied by delicate cellular membranes. Several such layers, 

 intimately blended together, form the membrane. 



Beneath the arachnoid, and 

 constituting a part of it, there 

 is a reticulum of subarachnoid 

 trabecules. These trabeculae 

 consist, as in the case of 

 the arachnoid proper, of fine 

 fibrous tissue, but the inter- 

 trabecular spaces, instead of 

 being occupied by cellular 

 membranes, contain cerebro- 

 spinal fluid. The trabecular 

 reticulum connects the arach- 

 noid with the subjacent pia 

 mater. 



Spinal Pia Mater. — ^This 

 is the most internal mem- 

 brane of the spinal cord. 

 It is very vascular and 

 closely invests the cord, 

 the substantia gelatinosa 

 externa or glial sheath 

 being situated immedi- 

 ately beneath it. Su- 

 periorly it is continuous 

 with the cranial pia mater, 

 the conus medullaris over 



.. Dura Mater (Theca) 



. lanea Splendens 



Ligamentum Denticulatum 



_L Pia Mater 



Anterior Nerve-Root 



Fig. 546. — A Portion of the Spinal Cord 

 (Anterior View). 



(The Theca has been laid open, and the 

 Arachnoid Membrane removed). 



and interiorly it is prolonged from 

 the upper half of the intrathecal part of the filum terminale. On 

 either side it forms tubular sheaths for the ventral and dorsal roots 

 of the spinal nerves, which blend with the sheaths of the nerves. 



Along the course of the ventral median fissure of the cord it sends 

 a vascular fold into that fissure. Along the course of the dorsal 

 median fissure, over which it passes, the neuroglial septum occupying 

 that fissure is attached to it. From the deep surface of the pia 

 mater several septa are prolonged into the cord, which carry with 

 them portions of the glial sheath. 



The pia mater is separated from the arachnoid by the subarach- 

 noid space, and opposite the dorsal median fissure of the cord it is 

 connected with the dorsal part of the arachnoid by the septum 

 posticum. 



