472 



HUMAN ANATOMY. 



Fig. 493. 



regions of the body and to a certain extent into the head region, there being, how- 

 ever, in this last region a considerable area in which the muscle-plates are unrepre- 

 sented. 



Throughout this area of the head region muscles occur which arise in relation 

 to the branchial arches and, accordingly, in a much more ventral position than the 

 mesodermic somites. Furthermore, these muscles are supplied by branches from 

 the mixed cranial nerves, arising from cells situated in a portion of the medulla 

 oblongata which is comparable to the lateral horn of the spinal cord and con- 

 stituting what are termed lateral motor roots, in contradistinction to the median 

 or anterior motor roots which supply the muscles derived from the mesodermic 

 somites. 



There are thus two sharply defined systems of musculature : the one, primarily 

 confined to the cranial region, is supplied by lateral motor nerves, and from its rela- 

 tion to the branchial arches may be termed the branchiomeric miiscidatiire ; the other, 



supplied by anterior motor nerves, is arranged 

 primarily in a series of segmental (metameric) 

 plates, and may be termed the metajneric tnus- 

 cidaturc. These two systems constitute the 

 first divisions in the morphological classifica- 

 tion of the musculature. 



The further subdivision of the branchio- 

 meric muscles is most conveniently made with 

 reference to the various cranial nerves by 

 which they are supplied. For the metameric 

 musculature a more complicated subdivision is 

 both necessary and convenient, and in the first 

 place it may be divided into the axial and 

 the appendicular musculature. For the latter 

 group, which includes all the muscles of the 

 limbs, a derivation from the mesodermic so- 

 mites seems probable, outgrowths from certain 

 somites extending into the limb-buds when 

 these develop ; but it has not yet been possi- 

 ble to demonstrate that this is the case, the 

 limb muscles really making their appearance 

 in an unsegmented mass of mesoblast in the 

 limb-bud which appears to have no connection 

 with the mesoblastic somites, these structures 

 apparently not being continued into the limb- 

 bud, but seeming to stop short at its base. In- 

 deed, it is quite possible that the limb muscles 

 should not be included under the metameric 

 musculature ; but until it is demonstrated that 

 their mode of development is not a secondary 

 condensation of the embryological history, it 

 seems preferable to retain them as members of 

 that group. 



The later development of the cranial mes- 

 oblastic somites is somewhat different from that of the others, and it is consequently 

 convenient to group the axial muscles derived from them by themselves. And since 

 the somites form in the embryo two clearly defined groups, it seems well to place 

 the derived muscles in two groups which may be termed respectively the orbital and 

 the hypoglossal groups. 



The remaining somites, which may be grouped together as the trunk somites, in 

 their later development undergo numerous modifications, some of which may be 

 regarded as fundamental and primarily affecting all of the series, and thus affording 

 a basis for a further subdivision. The most fundamental of these modifications is a 

 division of each somite into a dorsal and a ventral portion, corresponding respectively 

 to the primary divisions of the spinal nerves, and permitting the recognition of a 



Diagram showing grouping of head and trunk 

 myotonies. Ill, IV, VI, orbital group (supplied by 

 cranial nerves indicated by Roman numerals) rep- 

 resenting persisting first three cephalic myotomes; 

 XII, hypoglossal group, representing persisting 

 last three cephalic myotomes, intervening ones 

 having disappeared ;. i, i, i, i, i, first myotome of 

 cervical, Uhoracic, lumbar, sacral, and coccygeal 

 groups of trunk myotomes. Each myotome is di- 

 vided into dorsal and ventral segments. 



