292 



NERVE-TISSUE. 



sheaths are preserved and the myeline is absent, a faint trace only 

 of the axis-cylinder is discerned. We recognize the myeline 

 sheath with its oblong nuclei ; at pretty regular intervals it sends 

 out transverse septa through the myeline investment, the signifi- 

 cance of which will be spoken of later. The reticulum of the 

 myeline sheath is very distinct where the myeline has oozed out. 

 The axis-cylinder sheath can be recognized here and there, 



though it is often very diffi- 

 cult to distinguish it from 

 the stretched myeline sheath, 

 which may lie close to the 

 axis-cylinder. (See Fig. 123.) 

 (E) The Connective-tissue 

 Investments of the Brain and 

 the Spinal Cord. The brain 

 and spinal cord have three 

 membraneous investments 

 the dura mater, the arach- 

 noidea, and the pia mater. 

 The spaces between these are 

 filled with a varying amount 

 of cerebro-spinal liquid. The 

 space between the dura mater 

 and the arachnoid is called 

 the subdural space; that be- 

 tween the arachnoid and the 

 pia mater bears the name sub- 

 arachnoidal space, and is tra- 

 versed by trabeculae of con- 

 nective tissue, uniting the 

 membranes. In the spinal 

 canal this space is subdivided 

 into halves by the Lig. den- 

 ticulatum, and contains the 

 large blood-vessels at the base of the brain. A third space 

 between the pia mater and the surface of the brain may be pro- 

 duced artificially by the injection of liquids from without ; it is 

 called the epicerebral space. 



The dura mater of the skull represents the periosteum of the 

 cranial bones, while in the spinal canal there is a periosteal 

 investment of the vertebrae, in addition to the dura mater. It 

 is composed of very firm, dense interlacing fibers of connective 

 tissue. Its outer layer is well provided with blood-vessels enter- 



FIG. 123. WHITE SUBSTANCE OF THE 

 SPINAL CORD OF THE HORSE. LON- 

 GITUDINAL SECTION. 



A, axis-cylinder; AS, axis-cylinder sheath; 

 N, nucleus of the myeline sheath ; MS, mye- 

 line sheath, with oblong nuclei. Magnified 600 

 diameters. 



