Charles R. Bardeen 3 
not to the ingrowth into the limb of myotomes accompanied by nerves, 
but to the fact that a given region in the developing musculature is in 
the more direct path of fibres extending into the limb from one or two 
specific spinal nerves. ‘The number of spinal nerves contributing to the 
innervation of the inferior extremity in man varies from six to nine, the 
number contributing to the innervation of the musculature probably 
varies from five to eight. The number as well as the position of the 
spinal nerves serving to innervate a given muscle varies greatly in differ- 
ent individuals. 
With a few exceptions it is difficult or impossible to trace back to their 
origin from the plexus the fibres composing the nerve of supply of a given 
muscle in the inferior extremity in man. The path of fibre bundles in 
a nerve is quite different from that of the nerve fibres composing the 
nerve. The connective tissue which serves to hold together the nerve 
fibres and to distribute blood vessels to the nerve does not form continu- 
ous sheets about continuous bundles of nerve fibres. On the contrary it 
forms enveloping layers which are continued for but a short distance 
about a given group of fibres and then breaks up and becomes fused with 
similar enveloping sheets about other groups of fibres. The nerve fibres 
take a much more direct course in a nerve than any bundles that can be 
dissected from the nerve. A study of the origin of the branches of a 
nerve and the variation in the relation of these branches to one another 
makes it possible to construct a schematic cross section of a nerve trunk 
in which the relations of the nerve fibres in the trunk are more accurately 
revealed than in mere dissection of the branches back into the component 
fibre bundles of the nerve. On pages 308, 316, and 322 I have shown such 
schematic diagrams of the femoral, obturator and sciatic nerves. The 
nerve fibres of contiguous areas may branch off in a common trunk, but 
nerve fibres in discontinuous areas never do. On Plate III I have shown 
schematically the probable regions occupied by the fibres destined for the 
chief branches of the main nerves of the inferior extermity in the nerve 
trunks near the pelvis at the period when the segmental relations of the 
spinal nerves to the limb are becoming established. 
In the adult nerves variation is frequent and extensive. The main 
nerve trunks are fairly constant in position, the greatest variation being 
found in the course of the peroneal nerve in the thigh. This nerve is 
frequently separated from the tibial nerve by a part or the whole of the 
piriformis muscle. In one instance I have seen it separated by a part of 
the short head of the biceps, p. 293. In the embryo the peroneal and 
