AFFERENT AND EFFERENT NERVES. 235 



spinal centre, and extended thence through the body to the 

 muscles, sensible parts, and other organs laid under their con- 

 trol. The nerves constitute the medium of communication 

 between these distant parts and the nervous centre. One order 

 of nervous filaments, named afferent or centripetal, carry im- 

 pressions towards the nervous centre ; another, the efferent or 

 centrifugal, convey motorial stimuli from the centre to the 

 moving organs. The nerves are therefore described as inter- 

 nun cial in their function ; the cerebro-spinal centre receives 

 the impressions conducted to it by the one order of nerves, 

 and imparts stimuli to the other, while it renders certain 

 of these impressions cognisable in consciousness, and com- 

 bines in due association, and towards a definite end, move- 

 ments, whether voluntary or involuntary, of different and often 

 distant parts.* 



Besides the cerebro-spinal centre, the term centre is some- 

 times applied to certain bodies named ganglia, connected with 

 nerves in many situations. Ganglia are of small size, yet 

 possess the same elementary structure as the brain, and seem 

 to have the same relation as the cerebro-spinal centre to the 

 nervous filaments with which they are connected. Hence it is 

 believed, though grounds of doubt exist, that the ganglia are 

 nervous centres to which impressions may be referred, and 

 from which motorial stimuli may be emitted in a reflex man- 

 ner, and such operate independently of consciousness and of 

 the intervention of the will. 



The nerves fall under two heads, the cerebro-spinal and 

 the ganglionic nerves. The cerebro-spinal nerves are trans- 

 mitted in particular to the skin, the organs of the senses, 

 and such parts as are endowed with manifest sensibility, 

 and to muscles placed more or less under the dominion of the 

 will. They are connected in pairs to the axis of the cerebro- 



* Sharpey in ' Quain's Anatomy,' vol. i. p. 169. 



