BELATION BETWEEN STRUCTURE AND FUNCTION. 7 



tion of the nerve-cell branches within the brain and spinal 

 cord themselves. 



It may be conceded, therefore, that the nerve-cell and 

 fibre are trustworthy guides to the plan of the segmental 

 architecture of the body and of the limb. Various other 

 indices have been used, such as the bones, muscles, &c. 

 The former of these led to the view (Goodsir) that the 

 number of digits of the limb told the number of meta- 

 meres in the limb. As to the muscles, the examination 

 by the nerve-distribution method shows that even the 

 most homogeneous-seeming muscle may be traceable to 

 belong to two and three segments, and be combined from 

 elements of each. It is a point upon which the embryo- 

 logist is undecided whether the myotoms grow out into 

 the limb-bud ; so that embryology is evidently quite 

 unable to say even should the myotoms be proven, as is 

 likely, to grow out into the limb, and so give rise to its 

 musculature, what particular part of the limb musculature 

 is the outcome of any one particular myotom. 



What is the result of the analysis by the nerve-distribu- 

 tion method? As regards the muscular part of the 

 metameres some of the results are indicated in the 

 accompanying synopses. The synopses include the 

 range of individual variation, so that although the e.g. 

 flexor sublimis digitorum muscle is considered as quadri- 

 segmental, i.e., belonging to four body segments of the 

 brachial region in the species, it in any individual of the 

 species will belong only to three. To this matter of 

 individual variation I will return later. 



The topographical representation of the various elemental 

 movements of the limb in the chain of segments of the 

 spinal cord is always arranged in a definitely ordered 

 sequence. This sequence is exhibited in the accompany- 

 ing diagram : — 



