chap, iv.] SPINAL NERVES! RECAPITULATION. 145 



hypothesis that both forms are descended from a common ancestor, 

 that such changes and renumbering of whole nerves must have 

 happened, though there is evidence to shew that this may happen 

 piecemeal, as in cases given. 



Of course in speaking of such changes among the vertebrae it 

 will not be forgotten that partial changes occur too, but there 

 is still greater Discontinuity in their case than in that of the 

 nerves. 



But that there is Discontinuity in the case of nerves also is 

 clear ; for a given fibre, supplying a given muscle, must leave the 

 spinal cord either by one foramen and one spinal nerve, or by 

 another. Conversely the ?ith motor nerve must supply either one 

 muscle or another, and the transition between the two, however 

 finely it may be subdivided, must ultimately be discontinuous in 

 the case of individual fibres. It would be interesting to know to 

 what extent fibres vary in bundles, but this can hardly be deter- 

 mined. 



There is, however, some evidence that the group of fibres 

 supplying a limb does to some extent vary up and down the series 

 as a group, though much rearrangement may occur also within the 

 limits of the group itself. 



Lastly, there is important evidence that Variation in other 

 parts may be correlated with change in the ordinal positions at 

 which nerves with given distributions emerge from the spinal 

 cord. With Variation in the ordinal positions at which the nerves 

 come out, change in other parts, notably in the ribs, may happen 

 too ; so that we may say that in a sense there may be, at least 

 within the limits of single species (see cases Nos. 24, 65 and 

 71), a correlation between the apportionment of their functions 

 among the nerves and the contour of the body, both changing 

 together, the ribs rising and falling with the rise and fall of the 

 brachial plexus. The nerves do not merely come out through the 

 foramina like stitches through the welt of a shoe, the shape of the 

 shoe remaining the same wherever the threads pass out. The 

 arrangement is, rather, like that of the strings of such an instru- 

 ment as a harp or piano, in which there is a correlation between 

 the curves of the frame and the positions of the several notes: so 

 long as the frame is the same, the strings cannot be moved up or 

 down, the instrument still retaining the same compass and the same 

 number of notes. 



B. 



10 



