GENERAL CONSIDERATION OF THE VOLUNTARY MUSCLES. 473 



Ventral divi- 

 sion of spi- 

 nal nerve 



Fig. 494. 



Dorsal division of spinal nerve 



Dorsal group 



Hyposkeletal 

 group 



Obliquus 

 group 



Rectus group 



Diagrammatic cross-section of body, showing primary groups oi 

 trunk muscles. 



dorsal and a ventral group of trunk muscles. The portions of the ventral divisions 

 on either side of the mid-ventral line separate to form a subordinate group of mus- 

 cles which may be termed the rec- 

 tus group (Fig. 494), the more 

 lateral portions giving rise to a 

 group which, from the prevailing 

 oblique course of its fibres, may 

 be termed the obliquus group; 

 and, finally, from the more dor- 

 sal portions of the ventral muscu- 

 lature there are developed in cer- 

 tain regions of the body muscles 

 which lie ventral to the bodies 

 or processes of the vertebrae, and 

 may be termed the hyposkeletal 

 muscles, in contrast to the re- 

 maining musculature which ex- 

 tends between the skeletal ele- 

 ments or lies dorsal to them, and 

 hence is termed the episkeletal mus- 

 culature. 



To sum up the classification 

 proposed it may be represented in the following manner : 



I. Branchiomeric Muscles. 

 II. Metameric Muscles. 



A. Axial Muscles. 



1. Orbital muscles. 



2. Hypoglossal muscles. 



3. Trunk muscles. 



a. Dorsal, 



b. Ventral. 



a. Rectus set. 



b. Obliquus set. 



c. Hyposkeletal set. 



B. Appendicular Muscles. 



Nerve-Supply. — The segmental regularity of the mesodermic somites is but 

 slightly retained in the adult, numerous modifications, such as fusion, tangential and 

 longitudinal splitting, migration, and even obliteration taking place in them to pro- 

 duce the various muscles of the adult. The various modifications have not in all 

 cases been traced, but a study of the nerve-supply of a muscle gives in many, if not 

 all, cases an important clue to its origin. This depends upon the fact that the 

 muscles may be regarded as the end-organs of the motor nerves, and that the seg- 

 mental relation established in the embryo between a nerve and the muscle-tissue 

 derived from a given mesodermic somite is not disturbed in later development, no 

 matter what changes of relation the muscle-tissue may undergo. Thus, when a 

 muscle, such as the rectus abdominis, is found to be supplied by a number of spinal 

 nerves, it is because it has been formed by the fusion of portions of a corresponding 

 number of mesodermic somites ; and when a muscle, such as the latissimus dorsi, 

 lying mainly in the lumbar region, is found to be supplied by a cervical nerve, it is 

 becavise it has wandered from its original point of formation in the cervical region. 



Variations in the nerve-supply are occasionally seen, especially in the limb mus- 

 cles ; but it seems probable that such variations are only apparent, the nerve-fibres 

 supplying the muscle being in all cases strictly equivalent, arising from the same 

 region of the spinal cord, even although they may pursue in different individuals 

 somewhat different paths in order to reach their destination. Thus, a muscle which 

 normally is supplied by fibres from the median nerve may sometimes be found to be 



