NEEVE-TISSUE. 293 



ing the skull-bones j while the inner layer, that which alone 

 forms the dura mater of the spinal cord, has a comparatively 

 scanty supply of blood-vessels. The inner surface of this mem- 

 brane is lined with a delicate layer of endothelia. 



The arachnoidea is composed of much more delicate bundles 

 of connective tissue than the dura mater (see Fig. 55, page 160), 

 and probably contains neither blood-vessels nor nerves. Both 

 its surfaces are covered with endothelia. It sends numerous 

 trabeculae in an oblique direction into : 



The pia mater, which is also constructed of delicate inter- 

 lacing bundles of connective tissue and supplied with numerous 

 blood- and lymph- vessels and nerves. The prolongations of the 

 pia mater into the brain and spinal cord convey mainly capil- 

 lary blood-vessels, as the division of the arteries into capillaries 

 takes place before their entrance into the nerve-centers. The telae 

 choroideae are freely vascularized formations of the pia mater ; 

 their blood-vessels are coiled in bundles, and produce the lobules 

 which are covered with large, partially ciliated, endothelia, often 

 containing pigment and fat-granules. 



(F) The Ganglia. Nerves of the cerebro-spinal system are 

 provided in certain localities with spindle-shaped, or globular 

 or crescent-like, enlargements, the ganglia, which consist of an 

 accumulation of ganglionic elements, greatly varying in size, 

 and arranged either in rows or in clusters, which are more 

 numerous at the periphery of the ganglion than at its center. 

 The manner in which nerve-fibers are connected with the gan- 

 glionic elements has not yet been elucidated. Each ganglion 

 is ensheathed by a connective-tissue capsule, and divided by 

 septa of connective tissue into smaller portions ; sometimes 

 every single large ganglionic element is inclosed by a connective- 

 tissue sheath, the connective tissue being always freely sup- 

 plied with blood-vessels. 



The ganglia of the sympathetic nerve system contain smaller 

 ganglionic elements and numerous globular bodies, exhibiting 

 the features of lymph-corpuscles or nuclei rather than those of 

 ganglionic elements. The ganglionic bodies are, as a rule, mul- 

 tipolar. In those of the frog the central axis-cylinder was found 

 to be surrounded by a delicate spiral fiber (Beale), which is also 

 considered to be an offshoot of the ganglionic element. J. Arnold 

 claims that the straight central fiber comes from the nucleolus of 

 the ganglionic body, while the spiral fiber originates from its 

 periphery. All these assertions have been contradicted, and need 

 further proof before they can be received. 



