ON THE MAMMALIAN NERVOUS SYSTEM. 335 



If we select out of the whole table those readings which were obtained with fits of 

 a unilateral character, thus disregarding the whole of the larger numbers, we find 

 that the average amounts to 20, this being very much below the average unilateral 

 effect in the cord. (See Chapter VII.) 



It might be imagined that this difference in amount between the nerve and cord 

 effect was entirely connected with the difference in sectional area and thus in resist 

 ance ; that this is not the case is shown by the following considerations : 



(a.) The total resistance in connection with the preparation is made up of the 

 electrodes with their cables and the tract of tissues, of these the resistance of the 

 cables is far in excess of that of the tissue, and hence any difference in resistance 

 between the spinal cord and nerve preparations due to the sectional area of the tissue 

 is but a small fraction of the whole. 



(b.) The absence of effect in the capillary electrometer, when connected with the 

 nerve, shows that any unusual resistance which may be offered by the nerve as com 

 pared with the cord is not the cause of the diminution, since the amount of the 

 electrometer movement is unaffected by changes of resistance. 



(c.) The sectional area of the nerve of an adult large Cat is nearly as great as that 

 of the dorsal cord of a young animal or a small Monkey, yet the effect is small in the 

 former and large in the latter structure. 



(d.) As will be seen in Chapter VII., on bilaterality of representation, it is possible 

 to obtain large effects in only one-half of the cord, split longitudinally. This portion 

 has apparently less sectional area than the sciatic nerve, but the electrical change in 

 it, as will be seen by reference to the tables, is far in excess of the nerve change. 



(e.) The change in the cord is undoubtedly one connected with the fibres of the 

 pyramidal tracts, it is therefore with reference to their sectional area as compared 

 with the area of the motor nerves in the sciatic nerve that any criticism on this head 

 should be directed. There is, however, every reason for supposing that the pyramidal 

 tracts in the dorsal region are smaller in cross section than the sum of the anterior 

 roots of the lumbar plexus. 



These considerations serve to emphasize the conclusion to which, as it appears to us, 

 the foregoing results tend, viz., that the extraordinary difference in quantity between 

 the electrical effect in the cord and in the sciatic nerve when the cortex is excited, 

 must be attributed to an alteration in the quality and quantity of the nerve impulses 

 in their passage from the cord into the nerve, and that the structure of the spina 

 centres, through which the impulses must necessarily pass to reach the issuing nerves, 

 so influences the transmission as to cause this striking change. This view is 

 strengthened by the remarkable confirmation which is given to it by experiments in 

 which both the spinal cord is directly excited and the centres in it discharged reflexly 

 by stimulating its posterior roots. It will be seen (Chapter X., p. 478, and Chapter 

 XL, p. 494) that under these circumstances the electrical effects produced in the sciatic 

 nerve are extremely small and resemble in amount those above referred to. 



