Metazoa Rana. 3 1 7 



least two other kinds, viz. impulses governing the action of 

 glands, and therefore called secretory, and those which 

 regulate the phenomena of growth and nutrition of tissues 

 generally ; to these latter the name of trophic has been given. 

 It is as yet doubtful whether secretory and trophic impulses 

 pass by special nerves devoted to the transmission of such, 

 or travel by nerves which are under other circumstances 

 intrusted with the carriage of motor impulses. We know, 

 however, that sensory impressions do have special nerves 

 devoted to their transport, while motor nerves carry im- 

 pulses which stimulate to action the muscles to which they 

 are distributed ; hence the names of sensory and motor 

 applied to the posterior and anterior roots of the spinal 

 nerves to indicate that messages from the outer world travel 

 to the central nervous system by nerve-fibres which enter 

 the cord by the posterior root, and that messages from the 

 central nervous system pass outwards to the muscles (glands 

 and tissues generally) by the anterior roots. Sensory fibres 

 are sometimes spoken of as afferent, and motor fibres as 

 efferent. 



From analogy with the higher animals it is probable 

 that the cerebrum has to do with the originating of volun- 

 tary actions and with the analysing of sensory impressions. 

 The cerebellum is probably the centre for the co-ordinating 

 of muscular movement, while the optic lobes and the 

 olfactory lobes have to do with the two important sense- 

 organs which the nerves originating from them supply, 

 namely, the eye and the ear. 



The spinal cord in the higher animals is, in all prob- 

 ability, chiefly a path for the transmission of impulses from 

 the brain outward or from the nerves inwards, though it no 

 doubt also has to do with the government of local move- 

 ments or simple organic changes not requiring the special 

 interference of the brain. These are known as reflex 

 actions, and consist essentially of three separate phenomena 

 viz. (i) the transmission of a sensory impression along an 



