THE BRAIN AND SPINAL CORD 109 



tracks along which the sprouting ends of the cut central portions 

 of the axon will grow, and the (le\eloping new axons will find their 

 ])ro])er terminations. It is \ery important to have no scar tissue 

 formed at the lesion where the cut ends of the injured nerve are 

 brought together. Regeneration is more rapid in young animals than 

 in old and also more rapid in warm-blooded types. Degeneration 

 proceeds centrally in some cases, and involves cytons and dendrites. 

 Xo regeneration of such cells takes i)lace. Nor is there complete 

 regeneration in the central nervous system. The phenomenon of 

 \Yallerian degeneration has been of great value in aiding the deter- 

 mination of the central origin and ])eripheral termination of groups 

 of nerve fibers. An experimental lesion is made involving a small 

 area in the spinal cord. Proper time for degeneration of the nerve 

 fibers concerned is permitted to elapse. The animal is then killed. 

 The cord is removed and serial sections made following the proper 

 technicjue devised for the purpose. Disintegrating fibers and cytons 

 have a distinguishing ai)i)earance and enable the investigator to 

 follow the route of the degeneration and thus determine the origin 

 of the fibers occupying the region of the cord involved in the 

 experimental lesion. 



THE BRAIN AND SPINAL CORD. 



Both the brain and sjMual cord are suspended within the carti- 

 laginous or l)ony ca])sule by several connective-tissue membranes. 

 The outermost, the dura mater, is a thick fibroelastic connective 

 tissue attached to the bony or cartilaginous capsule. A narrow 

 cleft, the subdural space, contains fluid and separates the dura 

 from an innermost membrane, the arachnoid, a thin connective-tissue 

 membrane. Connecting strands from the arachnoid to the dura mater 

 divide the subdural space into compartments, others join it to an 

 innermost membrane, the pia mater, immediately surrounding the 

 brain or cord. The spaces of the arachnoid are filled with fluid, 

 the cerebrospinal fluid, a term applied also to the fluid within the 

 ventricles of the brain and the canal of the spinal cortl. 



The brain like the spinal cord has a region of gray matter occupied 

 by cytons, nerve processes, and neuroglia; and a white matter 

 occupied by the processes and neuroglia only. Among the lower 

 forms the cytons are fewer. The cerebrum and cerebellum of 

 mammalian brains may be recognized by the type and arrangement 

 of cells; in the cerebral cortex the pyramidal cells are characteristic, 

 and in the cerebellar region the Purkinje cells are diagnostic features. 



