788 



THE SPINAL CORD. 



segments, then the perikarya of the afferent root cells of each segment lie 

 uncommingled with other nerve cells, and form the spinal ganglia (right and 

 left) of that segment. And the branches thrust by the cells composing each 

 spinal ganglion into the spinal cord, remaining unmixed with others, form a 

 pure collection of afferent nerve fibres, known morphologically as the dorsal 

 spinal root. This collection contains all the fibres from the sense organs in a 

 segmented area of the body surface, which may be called a segmental skin- 

 field (Fig. 354). It contains also all the fibres from the sense organs in the 

 muscular and visceral apparatus belonging to the same segment. Similarly, 

 the afferent root cells of the fifth and eighth and intermedium cranial 

 nerves intrude nerve fibres as unmixed afferent roots into the brain. In 

 a few but important exceptions, e.g. the vago-glosso-pharyngeal roots, the 

 afferent fibres are commingled with efferent. The primary afferent path 



ffeact 



FIG. 354. Diagram of the arrangement of segmental skin-fields in the hind limb 

 of the monkey. The numbers indicate the fields of the spinal nerves, thoracic 

 and post-thoracic, a./, the axial line of the trunk, whether dorsal or ventral ; 

 a.Z, the axial line of the limb, whether dorsal or ventral. The " overlap- 

 ping " of the fields is not represented. 



for olfaction is peculiar, in that the bodies of its nerve cells are not 

 aggregated, but, maintaining a very primitive disposition, 1 lie scattered 

 severally among cells of non-nervous type in the olfactory region of the body 

 surface. 2 Their proximal branches are, however, before entering the central 

 nervous system, collected into pure afferent roots, the olfactory nerves. 



Recurrent sensitivity. Magendie's later experiments led him to the 

 view that the ventral root has some sensory fibres in it, derived from those 

 comprising the dorsal root. Denied by many, 3 this has been upheld by 

 Bernard and Schiff, and the place of rebending of the sensory fibres has been 

 referred to a region of the peripheral nerve, some little way beyond the place 

 of union of the two roots in the nerve trunk. Severance of the dorsal root or 

 of the neighbouring dorsal roots excludes the phenomenon. Schiff noted, 



1 v. Lenhossek, "Sensibl. Fasern d. Regenwurms, " Arch. /. mikr. Anat., Bonn, 1892, 

 Bd. xxxix. S. 102. 



2 Grassi and Castronovo, ibid., 1889, Bd. xxxiv. S. 385. 



3 J. Mtiller, Tiedemann, Van Deen, Stannius, Retzius, and Backer, early repeating the 

 Magendie experiment, do not mention it ; Longet, Chauveau, and others mention it to 

 deny it. 



