CENTRAL NERVOUS SYSTEM. 641 



experiments and observations of Sir Charles Bell (1811) and Majendie (1822) 

 showed that sensation followed the stimulation of the central ends only of the 

 dorsal nerve-roots, and that direct contractions of the skeletal muscles occurred 

 only when the peripheral portions of the ventral and lateral roots were stim- 

 ulated. 



It had previously been shown by Hales and Whytt (1768) that even 

 though both roots were intact, destruction of the spinal cord prevented the 

 excitation of the dorsal roots from causing a reflex response, and hence the 

 cord was to be regarded as forming part of the pathway. Moreover, it had 

 been shown by the earlier investigators, before Bell, that the excitation of the 

 ventral roots produced a response. Brown -Sequard 1 showed that section of 

 the (last six thoracic and first two lumbar) dorsal roots caused (in guinea-pig, 

 rabbit, and dog) a vascular dilatation and a rise of 1 to 3 C. in the hind 

 limbs. Strieker showed that stimulation of the peripheral ends of the cut 

 dorsal nerves caused a rise in the temperature of the foot ; and Morat showed 

 that stimulation of the peripheral end of a cut dorsal root produced vaso- 

 dilatation. The studies in the degeneration of the nerve-fibres 2 show a small 

 group in the dorsal root which, upon section of the root between the ganglion 

 and cord, degenerates toward the periphery and remains intact toward the 

 cord a behavior which is precisely opposite to that which occurs in the case 

 of the fibres taking origin from the spinal ganglion-cells. 



Finally, van Gehuchteu and others have shown, that in histological prep- 

 arations (chick), these fibres can be traced through the ganglion itself (see Fig. 

 163). In the dorsal roots of the lumbar region of the monkey, Sherriug- 

 ton 3 was unable to find any efferent fibres. The connection of some of the 

 ventral roots with sympathetic ganglia was established by Budge (1851), and 

 physiological as well as histological observations show that the further con- 

 nection of these ganglion-cells with the elements which they ultimately control 

 is in many instances by way of the peripheral plexuses. 



Classification of Nerve-elements. In accordance with this arrangement 

 of the nervous system, the elements which compose it fall into three groups : 

 (1) The afferent cells, those whose function it is to convey impulses due to 

 external stimuli from the periphery, including the muscles and joints, to the 

 central system. The expression " external stimuli " is in this case intended to 

 include also such stimuli as act within the tissues of the body, for example, 

 those acting on tendons and muscles, and affecting the afferent nerves which 

 terminate in them. (2) The central cells, those the neurons of which never 

 leave the central system, and the function of which is to distribute within this 

 system the impulses which have there been received. (2) The efferent cells, 

 or those the neurons of which pass outside of 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 elements the cell-bodies of which lie 



1 Gazette medicale de Paris, 1856. 



3 Gad and Joseph : Archivfur Anatomic und Physiologie, 1889. 

 3 Journal of Physiology, 1895, vol. xvii. 

 41 



