496 MESSRS. F. GOTCH AND V. HORSLET 



Although the cross sectional area of the root is much smaller than that of the cord, 

 yet the effect in the fibres of the root (C) is greater than that in the fibres of the cord 

 (A), suggesting that there is less nerve energy after propagation through the afferent 

 side of the centre. It seemed at first sight not improbable that the effect at A pro- 

 duced by the passage of nerve impulses up the direct channels which join A with C, 

 plus additional nerve impulses evoked by a discharge of central activities, would be 

 very large. It would seem, however, if the electrical indications are to be relied upon, 

 and we see no reason to discredit them, that the impulses which enter the central 

 structures from Ex. suffer a diminution which more than counterbalances any such 

 addition. We are, however, well aware that the reduction may be due to a spread 

 of nerve impulses in their passage through the afferent side of a centre ; and since a 

 similar reduction occurs both in the case of impulses descending the pyramidal tracts, 

 and emerging in the roots, as well as in those which as described are reflex in character, 

 we incline to the opinion that ascending nerve impulses do suffer a diminution in 

 intensity in their passage through the afferent side of a centre. 



With reference to the impulses descending the pyramidal tracts, whether originating 

 in the cortex cerebri or the fibres of the corona radiata, the facts given in Chapters V. 

 and VI. suggest that a very considerable reduction occurs before they emerge in the 

 anterior roots. 



In this connection we would here draw attention to some experiments which we 

 have made to record the muscular contraction evoked by excitation of the spinal 

 centres, and particularly to measure the time between excitation and the commence- 

 ment of the muscular contraction. The period of delay or latency thus estimated 

 was ascertained by the use of TIGERSTEDT'S signal method, as described in Chapter TIT., 

 Section 3, and in Chapter VIII., Section 1. In the latter place we have indicated 

 at length the method we employed for the measurement of the rate of transmission 

 of the excitatory effect in the cord fibres, which we found to be about 39'5 metres 

 per second. We have now to point out the reasons which led us to measure the 

 loss of time in a Mammalian spinal nerve centre. It appeared to us that this 

 loss of time which the excitatory condition surfers in going through a spinal nerve 

 centre would not be the same if the path of its entry into the centre is different. 

 There are two modes of entry with which we are familiar. These are (1) the 

 termination of the pyramidal fibres in the nerve centre, (2) the termination of the 

 afferent channels in the posterior root in the nerve centre. 



Now, as stated on p. 481, the latter has frequently been estimated, but EXNER has 

 attempted the measurement of the former, and not with a very definite result. 

 These periods of time have not to our knowledge been measured in the Cat, and, as 

 we felt the importance of forming some idea of their value, we performed a few 

 experiments as follows. 



The problem in each case is shown in fig. 23. The nerve centre is at EA, E being 

 its efferent, A its afferent side, and f the field of conjunction. 



