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AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



d.r. 



the central system, and which carry impulses to the periphery. In this last 

 group, again, two minor divisions may be made, namely : (a) the efferent ele- 

 ments, the cell-bodies of which lie within the central system, as is the ease with 

 those giving rise to the ventral roots ; (b) those forming the peripheral ganglia 

 entirely outside of the central system — the sympathetic ganglia and the more 

 or less solitary cells which take pari in the formation of the peripheral plexuses. 

 It was Sir Charles Bell who also showed the motor character of the 

 ventral roots. Nevertheless the observation was soon made, that while 

 stimulation of the central end of the severed ventral root was always 

 without apparent effect, the stimulation of the peripheral end in addition 

 to the typical motor responses might .sometimes cause expressions of pain. 

 This latter result was obtained even when the mixed nerve trunk, beyond 



the union of the two roots, had Keen severed, so 

 that the only possible pathway for the impulses 

 was through the junction of the two roots to 

 the spinal ganglion, and so by the dorsal root 

 to the cord. It appears probable from studies 

 on the degeneration of the root fibres that the 

 peripheral axones of some afferent neurones on 

 their way to the meninges do run hack toward 

 the cord within the ventral root, and that it is 

 the stimulation of these fibres which gives rise 

 to the phenomenon of "recurrent sensibility" 

 as it is called. 



The "afferent neurones" (1) have their 

 cell-bodies collected to form the spinal gan- 

 glia. 1 The distal branches of these cells form 

 the peripheral sensory nerves, and the prox- 

 imal branches combine to form the dorsal 

 nerve roots. On entering the walls of the 

 spinal cord these latter fibres divide info two principal longitudinal branches 

 which lie about the dorsal horns and form the major part of the dorsal 

 columns. From time to time the ends of these branches, or their collaterals, 

 enter the gray matter of the cord. 



Thus, in a cross-section of the cord, the dorsal column and the gray matter 

 represent the localities where the axones of the afferent elements are found. 

 The afferent cranial nerves which can be homologized with the afferent 

 spinal nerves have a corresponding distribution in the bulb. 



The "efferent neurones" (3) have their cell-bodies only in the ventral 

 horns of the spinal cord, or the homologous localities in the bulb and brain 

 stem. Sec Figs. 88, 89, which show the part of the medullary tube divided 



1 Recent work on the spinal ganglia shows that in addition t<> the elements usually deserihed, 

 they probably contain cells, the axones of which are distributed entirely within the spinal gan- 

 glion, and also cells which send their branches to the distal side only of the ganglion. See 

 Dogiel : Anat. Anz. Jena, 1896, Bd. xii. 



Fig. 88.— Cross-sectioD in the cer- 

 vical region of a fetal human spinal 

 cord at tlir sixth week ; < 50 diameters 

 (V. Kolliker): <\ central canal; o. a., 

 groove separating the two laminae ;d. p., 

 dorsal lamina ; v, p., ventral lamina, in 

 which alone arc Located nerve-cells 

 the axones of which leave the central 

 system; d. r., dorsal root; v. r., ventral 

 root. 



