THE NERVOUS SYSTEM 461 



ganglion on the ventral root. Dorsal and ventral roots unite 

 outside the intervertebral foramen to form the ordinary mixed 

 spinal nerve. The functions of these two roots is quite opposite. 

 In the dorsal roots the impulses are passing from the periphery 

 to the centre ; they are afferent fibres, and from the fact that 

 they convey impulses which give rise to sensations of various 

 kinds, they are also known as 'sensory.' In the ventral roots 

 the impulses are passing from the centre to the periphery ; they 

 are efferent nerves, and, from the fact that the majority of the 

 efferent impulses result in movement, they are also known as 

 1 motor ' nerves. A portion of cord embracing a pair of spinal 

 roots is spoken of as a segment, and the spinal cord consists of 

 a series of such segments united end to end. 



Passing away from the spinal cord in company with the 

 ventral roots is a branch known as the white ramus communicans. 

 So soon as it gets outside the vertebrae it leaves the ventral root 

 and passes to a portion of the nervous system known as the 

 ' sympathetic.' The sympathetic system is frequently regarded 

 as a something quite distinct from the ordinary cerebro-spinal 

 system, and in function this is very largely the case ; but the 

 white ramus communicans serves to remind us that the two 

 systems are very closely linked, and under one central authority. 

 It is unnecessary here to follow the white ramus any farther ; 

 it will be fully considered with the sympathetic system. 



Function of the Spinal Nerves. — If the dorsal spinal roots be 

 divided, all parts supplied by them below the division lose sensa- 

 tion ; if the portion of nerve in connection with the spinal cord 

 be irritated, pain is produced. The spinal sensory fibres endow 

 the whole body with sensation, with the exception of certain 

 parts of the face. If the ventral roots be divided, all parts 

 supplied by the nerves below the seat of division suffer motor 

 paralysis ; if the cut end of the nerve still in connection with the 

 tissues be irritated, the muscles contract vigorously ; while if 

 the piece of nerve in connection with the cord be irritated, 

 nothing happens. In this way it is demonstrated that the 

 sensory impulses pass into, whilst the motor impulses pass out 

 of, the cord. Sometimes pain is felt when the motor roots are 

 divided, due to one or two branches of the sensory nerves finding 

 their way back into the cord by this channel. The phenomenon 

 is known as recurrent sensibility. 



Columns of the Cord. — If a cord be suitably prepared, a trans- 

 verse section (Fig. 137) shows it to consist of two similar halves, 

 united by a comparatively small central mass of tissue, through 

 the centre of which a minute longitudinal canal runs. The 

 halves are separated by fissures on the dorsal and ventral sur- 

 faces of the cord. The ventral median fissure is wide, and does 



