

THE CENTRAL NERVOUS SYSTEM. 47 



Regeneration still takes place if two nerves are divided and the 

 central end of one is connected with the peripheral end of the other, 

 provided that both nerves are either afferent or efferent. Thus, if the 

 vagus nerve and the cervical sympathetic nerve are divided and the 

 central portion of the vagus is sutured to the. peripheral portion of the 

 cervical sympathetic, regeneration will occur, and stimulation of the 

 vagal portion of the united nerve will produce the effects formerly 

 resulting from stimulation of the sympathetic nerve. This experiment 

 shows that the effects of stimulation of a nerve are really due to changes 

 in the nerve ending, and not to any specific change in the nerve itself. 



When nerve fibres are divided in their course in the brain or spinal 

 cord they undergo degeneration, but regeneration never occurs. 



SECTION II. 

 THE SPINAL CORD (SPINAL MEDULLA). 



The spinal cord consists of white matter (substance) and grey matter 

 (substance), the former lying superficially, the latter deeply. The 

 white matter is composed of medullated nerve fibres, running in a 

 longitudinal direction, and having no neurolemma. The grey matter 

 contains numerous nerve cells, arranged for the most part in two horns 

 (columns), an anterior and a posterior, in each of which the cells form 

 groups corresponding with the primitive segments of the body. 

 Numerous fine medullated nerve fibres run into the grey matter, and 

 terminate by forming arborisations in relation with the nerve cells. 

 These fine fibres are called collaterals, and they arise at right angles 

 from the medullated fibres of the white substance. The axons of each 

 segmental group of cells in the anterior column of grey matter form 

 an anterior root of a spinal nerve. The corresponding posterior nerve 

 roots are formed of the axons of the cells in the respective spinal 

 ganglia, and enter the spinal cord in the neighbourhood of the posterior 

 horn (column) of grey matter. The course taken by these fibres in 

 the spinal cord will be described later. 



The spinal cord consists of two symmetrical halves, separated by the 

 anterior median fissure in front and by a septum of pia mater called 

 the posterior median fissure (septum) behind. The grey matter forms 

 a crescent in each half in a transverse section, the convexity of each 

 crescent being towards the middle line and being connected with the 

 convexity of the crescent in the other half of the spinal cord by a 

 commissure of grey matter. The central canal, containing cerebro- 

 spinal fluid and lined by ciliated epithelium, lies in \his commissure. 



