68 Anatomy of the Rabbit. 



ganglion. Impulses passing through the dorsal root are centri- 

 petal or afferent in that they pass only in the direction of the 

 central nervous system, but they are also in many cases sensory 

 in that their effects may he consciously experienced. The most 

 characteristic sensory impulses are those which come from the skin. 

 In a similar fashion the impulses of the ventral root are centrifugal 



COMPOSITION OF ° r efferent > in that the >' P ass onl >" m a 



A SPINAL NERVE direction away from the central nervous 



system, and are in most cases motor in that 

 their effects are commonly observed as muscular contraction. The 

 two roots, however, unite immediately outside the spinal cord, and 

 subsequently re-divide in such a way that three nerves are formed, 

 each containing a proportion of both kinds of fibres. Two of these 

 nerves, known as the dorsal and ventral rami and then distributed 

 as somatic nerves to the body wall, while a third is distributed as a 

 visceral nerve, or ramus communicans, having important se- 

 condary connections in the sympathetic nervous system. Unlike 

 the somatic nerves, which take a direct course to their terminations, 

 the communicating rami of each side, unite in a position ventral 

 to the vertebral column to form a longitudinal sympathetic 

 trunk consisting of a connected series of ganglia. The latter 

 is similarly connected with a prevertebral series of the ganglia, and 

 through them with certain peripheral ganglia on the surface of 

 the visceral organs. This system forms the sympathetic division 

 of the peripheral nervous system. Its ganglia are nerve-cell 

 centres, and its fibres, afferent and efferent, are distributed both 

 to the visceral organs and to bloodvessels in all parts of the body. 

 It is difficult to determine what portion of a muscular con- 

 traction, even if considered to be purely voluntary, actually arises 

 from an impulse originating in the central nervous system. The 

 animal body, however, affords many indications of muscular 

 REFLEX ACTION actions as responses to previous incoming 



stimuli, without conscious experience as a 

 necessary factor in the result. Stimulation of the skin, in verte- 

 brates in which the spinal cord is divided, and thus separated from 

 the brain, is followed by muscular contractions, presumably by 

 direct connections of individual dorsal and ventral roots, or exten- 

 sion of the stimulus to neighbouring roots. This is known as 

 reflex action. Its nature and conditions can be determined by 



