4 14 



HISTOLOGY 



White External limiting 

 substance. membrane. 



in the pia mater. Between the two layers of the pia, as described by Stb'hr , 

 there is a wide space filled with cerebro-spinal fluid and traversed by many 

 strands and membranes which pass from one layer of the pia to the other. 

 These strands constitute the arachnoid membrane, so-called from its 

 cobwebby texture. Often the name is restricted to the subdural mem- 

 brane (following Henle), so that the spaces between the meshes of the 

 arachnoid are described as subarachnoid. They are preferably termed 

 arachnoid spaces and they are of great importance. The fluid which they 

 contain has access to that within the central canal of the cord and the 

 ventricles of the brain, through an aperture in the thin roof of the medulla 



oblongata. Whether the arach- 

 noid spaces open directly into 

 lymphatic vessels may be ques- 

 tioned, but undoubtedly they are 

 freely drained by the lymphatic 

 system. 



On either side of the cord, be- 

 tween the successive spinal nerves, 

 there is a frontally placed tri- 

 angular plate of fibrous connective 

 tissue, which passes from the pia 

 to the dura and serves to' support 

 the cord. The succession of these 

 pointed projections, with their 

 bases attached to the pia, consti- 

 tutes the denticulate ligament. 



White Substance. The white 

 substance of the cord consists es- 

 sentially of medullated nerve fibers 

 supported by a network of neu- 



roglia. Toward the outer surface, the neuroglia fibers become felted to- 

 gether, forming an external limiting membrane just within the pia mater 

 (Fig. 425); this is an ectodermal tissue, which must be distinguished 

 from the adjacent connective tissue penetrating the cord with the blood 

 vessels. Although in transverse sections the neuroglia fibers appear to 

 be radially arranged (Fig. 425), longitudinal sections show that they ex- 

 tend also up and down the cord (Fig. 426), and in fact they form a dif- 

 fuse syncytial network. The protoplasm of this network is characterized 

 by the presence of stiff neuroglia fibrils, imbedded in the peripheral 

 exoplasm, and passing freely from one cell territory to another. They are 

 well shown in specimens stained with Mallory's phosphotungstic acid 

 haematoxylin, and resemble the myoglia and fibroglia fibrils both in form 

 and staining reaction. 



Cross sections of 

 medullated 

 nerve fibers 

 consisting of 



Axis cylinder 



and 

 Medullary sheath. 



,. Neuroglia cells. 



(f >. v Connective tissue. 



Blood vessels. 



FIG. 425. FROM A CROSS SECTION OF THE HUMAN 

 SPINAL CORD IN THE REGION OF THE LATERAL 

 FUNICULUS. X 1 80. 



