RECURRENT SENSITIVITY. 79 i 



path. In two frogs, Birge l found 3781 and 5335 fibres in the afferent roots, 

 and 3528 and 4283 respectively in the efferent roots. When the cranial roots 

 are included, the excess of inferent channels over efferent channels becomes, of 

 course, immensely greater still. It has been pointed out that, so far as 

 examined, no spinal peripheral nerve trunk 2 of macroscopic size is made 

 entirely of efferent fibres, whereas nerve trunks entirely composed of afferent 

 fibres are numerous enough. 



Probably the nerve impulse travelling by the afferent root cell of the 

 adult, with its T-shaped process, does not traverse the perikaryon but 

 sweeps tangentially past it. That the perikaryon might be indifferent 

 for conduction in such cases, was urged in 1889 by Cajal 3 from the 

 histology of cells of the mesencephalic roof of the bird. An experi- 

 mental observation by Be the 4 (1896) seems to have placed this hypo- 

 thesis beyond doubt for certain cells of Carcinus (second antenna). 

 Observations by Steinach 5 confirm the same view for the cells of the 

 spinal ganglion. The conduction is, of course, ultimately dependent on 

 the cell body, since the cell branches cannot maintain their vitality in 

 its absence. 



The application of nicotin 6 to the spinal ganglion does not hinder 

 the conduction of impulses through it, as in sympathetic ganglia ; 

 in the latter there is a synapse 7 or linkage between two cells. There is 

 a difference in the answer obtained by different observers as to the 

 question of alteration of speed of travel of nerve impulses travers- 

 ing a ganglion. Wundt 8 found the reflex time markedly shorter 

 from the spinal root than from the nerve trunk peripheral to the 

 ganglion. Exner 9 found the negative variation travel with unaltered 

 speed through the spinal ganglion. Gad 10 found a longer delay in the 

 development of a respiratory reflex, when excited from the far side of 

 the vagus ganglion, than could be accounted for by the greater length of 

 nerve cord traversed. Moore and Eeynolds n have recently obtained 

 on the spinal ganglion results confirmatory of Exner. 



A nerve impulse traversing the afferent root cell passes when once 

 within the spinal cord to various distances and in various directions, 

 without leaving the root cell, and before impinging upon other nerve cells 

 at the first intraspinal synapses. It is not known in many cases whether 

 one afferent root cell may serve as entrant channel from sense organs 

 of different kinds, e.g. whether one afferent nerve fibre from the skin 

 is peripherally connected with tactual and with " cold " and " pain " 

 organs. Probably each fibre is connected only with one species of 

 sense organ. Similarly, it is not known whether one afferent root cell 

 embouches intraspinally into only one or into a number of functionally 



1 Arch.f. Physiol., Leipzig, 1882, S. 476. See also Stilling, " Untersuch.," Kassel, 1857. 



2 Sherrington, Journ. Physiol. , Cambridge and London, 1894, vol. xvii. The cervical 

 sympathetic appears to consist wholly of efferent fibres. Schiff, Langley, etc. 



3 Rev. trimest. d. histol., tomes iii. and iv. pp. 67-78. 



4 Arch.f. mikr. Anat., Bonn, 1897-98, S. 50, 51. 



5 Arch.f. d. ges. Physiol., Bonn, Bd. Ixxviii. 



6 Langley and Anderson, Journ. Physiol., Cambridge and London, 1892, vol. xiii. 

 p. 465. 



7 For a description of this, see C. Hiiber, Journ. Comp. Neurol., 1897, vol. vii. p. 72. 



8 "Mechanik d. Nerven," 1876, Abth. 2, S. 567. 



9 Arch. f. Physiol., Leipzig, 1878, S. 567; Sitzungsb. d. k. Akad. d. Wissensch., 

 Wien, 1877, S. 729. 



10 Arch.f. Physiol., Leipzig, 1889, S. 218. 



11 Fourth Internal. Cong. Physiol., reported in Journ. Physiol., Cambridge and London, 

 1898, vol. xxiii., Suppl. 



