42G THE ORGANS 



part of whose processes, at least, lie outside the central nervous sys- 

 tem, forming the peripheral nerves. II. Central or intermediate 

 neurones. These lie entirely within the central nervous system. 

 The peripheral neurones may be classified as follows: A. Afferent 

 peripheral (all the neurone bodies located outside the central nervous 

 system, except possibly some in midbrain roof). The afferent 

 peripheral neurones are (i) the cerebro-spinal ganglion cells, which 

 may be (a) somatic, (b) splanchnic; (2) cells in olfactory membrane, 

 retina, and possibly midbrain; (3) some sympathetic neurones 

 (splanchnic). B. Efferent peripheral nenrones. These are (i) cerebro- 

 spinal (neurone bodies located in ventral part of central nervous 

 system), (a) somatic (to voluntary striated muscles except (c)), (b) 

 splanchnic (sending axones to sympathetic ganglia) , (c) splanchnic to 

 voluntary striated branchiomotor muscles (p. 425); (2) sympathetic 

 (splanchnic, bodies in sympathetic ganglia, sending axones to smooth 

 muscle, heart muscle and glands). 



In the central nervous system the bodies and dendrites of the 

 neurones are usually aggregated within certain localities which, on 

 account of their appearance when examined in fresh condition, are 

 collectively termed the gray matter (substantia grisea). The gray 

 matter also contains, of course, the beginnings and endings of the 

 nerve fibres. The parts of the nervous system where the bodies 

 and dendrites are absent and which are composed (exclusive of the 

 neuroglia, blood-vessels, etc.) of the medullated nerve fibers are 

 collectively termed the white matter {substantia alba). 



The central nervous system may be divided into a segtnental part 

 and certain suprasegmental parts. The former is in more immediate 

 relation with the peripheral (segmental) nerves. It comprises the 

 spinal cord and basal part of the brain [segmental brain). It con- 

 tains all the bodies of the efferent cerebro-spinal neurones ami 

 practically all the terminations of the central processes (afferent 

 root fibres) of the afferent peripheral neurones. In it the gray matter 

 is internal, as a rule, and the white matter external. The supraseg- 

 mental parts comprise the expanded portions of the dorsal wall of the 

 neural tube already mentioned, namely the pallium, corpora quad- 

 rigemina, and cerebellum. These constitute the highest coordi- 

 nating centers of the nervous system. Here the gray matter is exter- 

 nal, constituting a cortex, and the white matter is internal. 



The grouping of peripheral neurones into ganglia and nerves 

 has already been mentioned. In the central nervous system more or 



