ELEMENTS OF THE CENTEAL NEKVOUS SYSTEM. 



505 



- - Splanchnic 

 efferent cell 



gf ---Somatic 

 wij&k!.. efferent 



;alated cell (rf); and it is customary to distinguish these latter elements (within 

 ;he central nervous system) as splanchnic efferent cells. It is, however, a matter 

 f fundamental importance- to recognise clearly that the real splanchnic efferent 

 ;ells, the homo- 



A B 



Roof Plate 



ogues of the 

 jomatic efferent 

 jells, are found in 

 ;he sympathetic 

 ganglia, and that 

 he elements to 

 .vhich this term is 

 isually applied are 

 n reality inter- 

 calated cells. 



Floor Plate NX ^.- ; :\Yf-v/. : //v! ; .v nucleus 



This account is 



it variance with the 



;ustomary descrip- 



lon of the develop- 



nent of the sym- 



aathetic system, 



iccording to which 



he cells of the sym- 



oathetic ganglia are 



>aid to be wholly 



lerived from the sen- 

 sory ganglia ; but it 



)ffers a reasonable 



explanation of the 



; acts (i.) that the cells FIG. 445. DIAGRAM OF A TRANSVERSE SECTION THROUGH THE LEFT HALF OP THE 

 NEURAL TUBE REPRESENTING Two STAGES IN THE DEVELOPMENT ov THE 

 EFFERENT NERVES, TO SUGGEST THE POSSIBLE ORIGIN OF THE CELLS OF THE 

 SYMPATHETIC GANGLIA BY MIGRATION FROM THE NEURAL TUBE. 



n the sympathetic 

 ganglia are of the 

 Afferent, and not of 



;he sensory, type, 



md (ii.) that the fibres from the central nervous system establishing relations with them emerge 

 ; dong the motor nerves. Moreover, the information brought to light by recent research in 

 embryology (Froriep, Kuntz, and others) affords positive evidence in support of this view. 

 Elliott, however, opposes this interpretation (Journal of Physiology, 1907, p. 438). 



Many, if not all, of the sympathetic cells are derived from the walls of the 

 neural tube, and they migrate along the pathways formed by the motor, rather 

 'ohan the sensory, nerves. In the case of the spinal medulla they pass out chiefly 

 'ilong the anterior roots, and from the brain along the motor nerves the oculo- 

 motor, and the motor divisions of the facial and vagus nerves. 



Nerve Components. From the statements in the preceding paragraphs it must 

 ->e evident that there are several varieties of afferent and efferent nerves respectively 

 ' 3ntering and leaving the central nervous system. The cells of origin of the efferent 

 aerves are all placed in the ventral part of the side wall of the neural tube ; and 

 for this reason this part of the wall becomes swollen at an early stage of develop- 

 ment (Figs. 445 and 446). It is called the basal lamina. Most of the cells that emit 

 ifferent fibres are situated in the sensory ganglia outside the central nervous system, 

 }o that their growth can have no direct influence upon the form of the neural tube ; 

 r>ut their central processes become inserted into the dorsal part of the side wall 

 3f the tube, which is called the alar lamina; and groups of intercalated cells 

 3ollect around the entering fibres to form receptive or terminal nuclei. The 

 ^owth of these terminal nuclei leads to an expansion of the alar lamina which is 

 inalogous to, but much less extensive than, that seen in the basal lamina. This 

 unequal swelling of the dorsal and ventral parts of each side wall of the neural 

 ^ube leads to the development of a longitudinal groove, sulcus limitans, as a 

 lemarcation between the alar and basal laminae. 



The nuclei of origin of the efferent fibres, which are found in the basal laminae, 

 may be divided into two (and, in some regions of the nervous axis, three) main 

 groups. There is first the group of large multipolar nerve-cells which emit fibres 

 30 innervate the ordinary striped voluntary muscles. This is commonly called 



