RELATION OF AFFERENT TO EFFERENT ROOTS. 797 



segment. But there are so many and such wide exceptions to this, that the 

 rule as a law can hardly be said to exist. In respect to the proximal joints of 

 the limbs, the muscles producing the chief movements in opposite directions 

 are mainly represented at different segmental levels of the cord. Thus in the 

 macaque the motor root cells for flexion at the elbow lie mainly in the fifth, 

 sixth, and seventh cervical segments, those for extension at the elbow, chiefly 

 in the seventh and eighth cervical and first thoracic segments. The motor root 

 cells for opposite movements of the small joints at the free ends of the limb 

 are, however, not thus segmentally separate, but lie commingled in the same 

 segments of the cord. There are reasons for thinking, moreover, that the 

 motor root cells for antagonistic movements, whether as in the latter joints 

 segmentally commingled, or as in the former segmentally apart, are contem- 

 poraneously influenced by any reaction that, e.g. from brain or cord, initiates a 

 co-ordinate movement in which they take part, the one set of cells being 

 subjected to pressor, the other to inhibitory influence. 



The fact that the ventral spinal roots and certain analogous cranial nerve 

 roots form the only channels by which nervous impulses pass out of the 

 cerebro-spinal organ to the extrinsic tissues, allows the efferent root cells, taken 

 together, to be considered as one great efferent path for all nervous reactions. 

 The telencephalon and diencephalon (primary cerebral vesicle) are the only 

 portions of the nervous system whose root cells are solely efferent. The whole 

 path is made up of the third, fourth, sixth, eighth, eleventh, and twelfth 

 cranial nerves, of the unganglionated root of the fifth cranial and of all the 

 ventral spinal roots. There are some minute nerve fibres in the sheath of the 

 ventral root, which do not take origin from intraspinal cells ; these fibres may 

 be sympathetic. As already mentioned, in the ventral roots of the lumbar 

 region of the cat, Schafer discovered nerve cells, and similar have been seen in 

 the same region in man. Whether these are afferent or efferent, and if the 

 latter, whether sympathetic, is not certain. 



Golgi discovered that from the axons of some spinal efferent root cells " side 

 fibres" are given near its origin. The function of these is still unknown, 

 though probably important. They may conduct centripetally into the axon. 1 



Restoration of movement after " nerve-crossing." — When two mixed nerve 

 trunks (A and B), supplying antagonistic groups of muscles, are cut and 

 " crossed," so as to secure union of the peripheral end of A with the central of 

 B, and vice versa, Flourens 2 concluded that full control and co-ordination 

 was regained by some central adjustment. R. H. Cunningham 3 has, in the 

 case of the fore-limb of the dog, re-examined this question carefully, and finds 

 that on cortical excitation, and also in intentional co-ordinated movements, 

 the co-ordination of the muscular contractions is not regained. 



Eelation of Afferent to Efferent Boots. 



Besides the fact that excitation of the ventral spinal root excites 

 contraction of the muscles directly through motor fibres, Bell and 

 Magendie and their followers showed that its excitation evokes no signs 

 of sensation and no reflex actions, and in this respect offers a second 

 difference from the ganglionated root. This " law " of Bell and Magendie 

 sheds light on the quality of the intraspinal arc connecting the afferent 

 nerves and roots with the efferent nerves and roots. It is evident that 

 the nexus between afferent channel and efferent is of a kind allowing 



1 See v. Lenhosselc, " Der feinere Bau d. Nervensystems," 2te Aufl. S. 129. He terms 

 them "axo-dendrites." 



2 "R.echerches exper. sur propria, et foiict. d. systeme nerveux," 1824, p. 272. 

 Cf. Rawa, Arch. f. Physiol, Leipzig, 1885, S. 296. 



3 Am. Journ. Physiol., 1898, vol. i. p. 240. 



