372 The Nerves and Muscles of the Leg 
Since the more complex muscles are usually capable of being analyzed 
into parts the structure of which resembles that of the simpler muscles, 
we shall consider here the development merely of several simpler types 
of muscle. The structural units of the more complex muscles develop in 
a similar manner. 
The nerve usually enters the anlage of a muscle near the center of the 
side toward the main trunk from which its special nerve arises, Fig. 7 a. 
The relation of the chief branch or branches of the nerve of the muscle 
to the fibre-bundles depends on whether the course of the muscle fibres 
is transverse to or parallel with the main trunk from which the nerve 
arises. 
Tf the fibre bundles of the muscle take a direction directly or obliquely 
transverse to the course of the main nerve trunk the nerve to the muscle, 
or its chief branches, usually passes for some distance across the fibre 
bundles about midway between the tendons and give off rami on each 
side from which in turn an intramuscular nerve plexus arises, Fig. 7 c-g. 
The direction of the course of the main nerve branches on or in an adult 
muscle of this nature indicates the course of growth of the muscle from 
the anlage in a direction transverse to the long axis of the muscle fibres. 
This growth is relatively slight in case of ribbon-like muscles, Fig. 7 ¢, 
somewhat greater in case of triangular muscles, Fig. 7 d, and extensive 
in case of muscles like the intercostal muscles and pennate or bipennate 
iuscles, Fig. 7, e, f. Two or more branches may enter muscles of these 
latter types at different levels from the main nerve trunk, dotted lines 
Fig. 7 e. 
It is at first difficult to recognize that in most fusiform muscles the 
distance from tendon to tendon along the course of the muscle-fibres is 
approximately equal. The course of the muscle-fibres in such muscles is 
diagrammatically represented in Fig. 7, g. It will be noted that the 
course of the chief branches of the nerve to the muscle is approximated 
midway between the tendons to which the fibre-bundles are attached. 
If the long axes of the fibre-bundles of a muscle are developed in a 
direction somewhat parallel with the course of the main trunk from 
which the chief branches to the muscle arise, the branches usually enter 
the proximal third of the belly of the muscle and extend distally parallel 
with the muscle fibres, at the same time giving off rami from which an 
extensive intramuscular plexus is formed. The course of the chief rami 
within a muscle of this type indicates the course of growth of the muscle 
in a direction parallel with the muscle fibres. See Fig. 7, h and i. 
Metameric segmentation in the innervation of the limb muscles is due 
ee 
