RELATION BETWEEN STRUCTURE AND FUNCTION. 15 



of individual variation in the movements produced by 

 excitation of the roots, and in the distribution of the 

 nerve-fibres composing a root to the muscles of the limb. 

 In experiments on the plexuses, it is rare to find two 

 consecutive individuals of the same species that possess 

 a similar root distribution. The plexus in any species 

 requires for description to be grouped into classes. For 

 my purpose I have found two classes enough, a class of 

 individuals in which the plexus is pre-fixed and a class in 

 which it is post-fixed. Thus, in some individuals, the 

 supinator brevis is innervated from the 6th and 5th 

 cervical nerves, in others from the 6th and 7th. In the 

 former case the plexus is of pre-fixed type, and when one 

 muscle or one part of the plexus is pre-fixed, all the rest 

 of it is pre-fixed, and conversely, when one part is post- 

 fixed, all is post-fixed. In the second instance given 

 above, the supinator brevis is post-fixed, that is, its nerve- 

 cells are fixed in segments further aboral in the series 

 than in the individuals with pre-fixed plexuses. Presum- 

 ably the muscle itself therefore is built of myotoms more 

 aboral in the series than in the muscle in the pre-fixed 

 individuals. But instead of two classes, there might be 

 made a very large number of classes were it worth while 

 to separately distinguish them, for between the extreme 

 cases of post-fixed type and the extreme examples of 

 pre-fixed type extend a numerous series of intermediate 

 individuals ; in fact, any and every intermediate degree of 

 type seems to exist. The extreme of post-fixed type 

 passes over into the comparatively rare individual with an 

 additional segment altogether, e.g., the cat with fourteen 

 ribs, which is not at all an uncommon find in the laboratory. 

 Never a year passes without our meeting with four or five 

 such. This frequency of individual variations exhibits in an 

 almost ludicrous aspect the view that each motor spinal 



