790 THE SPINAL CORD. 



muscles only. The visceral effects have been confirmed by another observer, 1 

 those on skeletal muscles have also been confirmed, 2 though it is stated 

 they must be regarded as the individual exception and not the rule. 3 

 Strieker 4 affirmed the demonstration of vaso-dilator fibres in the mammalian 

 dorsal (posterior) roots. Veias 5 and Joseph 6 state, in contradiction to the 

 original observations by Waller, 7 that in the cat and rabbit injury to the 

 dorsal root, proximal to its ganglion, causes peripheral degeneration of a 

 number of nerve fibres, which can be followed into the distal nerve trunk. 

 On the other hand, the original observations by Waller have been confirmed 

 by many, 8 both in the spinal region of outflow of fibres for the sympathetic 

 system 9 and in other regions. The position arrived at is therefore as follows : 

 In Amphioxus, where all the nerve cells of both spinal roots are intraspinal, 

 some of the fibres of the dorsal root may be efferent, though the majority of 

 them are certainly afferent. 10 In Petromyzon some n of the fibres of the dorsal 

 root spring not from cells of the spinal ganglion, but from intraspinal cells. 

 These may be efferent. Also, in Pristiurus some of the fibres of the dorsal root 

 arise from intraspinal cells, but in Myxine all arise from the extraspinal cells 

 of the root ganglion. In the chick a few fibres of the dorsal root arise from 

 multipolar intraspinal cells beyond all doubt, and in Rana peripheral effects 

 result on exciting the peripheral ends of the dorsal root. In the mammal, 

 efferent fibres appear to exist in certain dorsal nerve roots. 



The total entrant path furnished by the whole collection of the 

 afferent root cells is conveniently divisible by functional qualities into 

 eight groups. These are (1) from the retina ; (2) from the olfactory 

 surface ; (3) from the cochlea ; (4) from the vestibule and semicircular 

 canals ; (5) from the integument ; (6) from the skeletal muscles (in- 

 cluding their tendons) and from joints ; (7) from the gustatory surface ; 

 and (8) from viscera. 



The conditions attending the initiation of nerve impulses in the 

 fibres of this entrant path are examined in the articles on the several 

 sense organs. These fibres appear amenable to the same general 

 stimuli as are other nerve fibres. Though afferent, they are quite indis- 

 tinguishable in their electro-motive reactions and microscopic structure 

 from efferent fibres. Certain differences, probably quantitative rather 

 than qualitative, are, it is true, found between the excitability of afferent 

 and efferent nerve fibres. The main laws of " double conduction," rate 

 of transmission, etc., treated of in the section on general physiology 

 of nerve, apply, however, to them as to other peripheral nerve fibres. 



The total number of separate channels composing the spinal entrant path 

 is probably always greater than the number composing the total spinal efferent 



1 See Steinach, Arch. f. d. ges. Physiol., Bonn, Bd. Ixxi. S. 554. 



2 Wana, ibid., Bd. Ixxi. S. 555. 3 H. E. Hering, ibid., Bd. Ixviii. S. 6. 



4 With his pupils Gartner, Hasterlik, and Biedl. Morat and, more recently, Bayliss 

 (Proc. Physiol. Soc., March 1900) have recorded results corroborating Strieker's. Bayliss 

 obtained vaso-dilator effects in the hind-limb (dog) on stimulating electrically, mechanically, 

 or chemically the peripheral cut ends of the fifth, sixth, and seventh lumbar and first sacral 

 dorsal nerve roots. 



5 " Beitr. z. Anat. u. Phys. d. spin. Gangl.," Munich, 1883. 



6 Arch.f. Physiol., Leipzig, 1887, S. 296. 7 "Nouvelle methode, etc., "Bonn, 1852. 



8 Bernard, "Systeme nerveux" (dog), 1858, tomei. ; Kahler, Prag. mcd. Wchnschr., 1884, 

 S. 302 (dog); Singer and Miinzer, Denkschr. d. k. Akad. d. Wisscnsch., Wien, 1890, Bd. 

 Ivi. S. 570 (dog) ; Sherrington, Journ. Physiol., Cambridge and London, 1894, vol. xvii. 

 p. 211 (cat and monkey) ; ibid., 1897, vol. xxi. p. 209. 



9 Sherrington, op. cit. 



10 J. F. Heymans and 0. v. d. Stricht, "Sur le systeme nerveux de 1' Amphioxus et sur 

 la constitution des racines sensibles," Bruxelles, 1898. 



11 Freud, Nansen, Retzius, etc. 



