STRIATED OR VOLUNTARY MUSCLE. 467 



bands, the myomeres (Fig. 493), which consist of a dorsal and a ventral portion, and 

 which succeed one another regularly and segmentally throughout the entire length 

 of the trunk. The muscle-fibres of which each myomere is composed extend 

 from the intersegmental septum in front to that behind, having thus a regular antero- 

 posterior direction. In the lower vertebrates this condition persists with but little 

 modification throughout life, producing the flake-like arrangement of the muscles 

 characteristic of the fishes. In the higher vertebrates, however, numerous secondary 

 modifications supervene, whereby the myomeres ar^ broken up into individual mus- 

 cles, their original segmental arrangement becoming at the same time greatly ob- 

 scured, although it still persists in those regions in which the muscles are intimately 

 associated with segmental skeletal structures such as the vertebrcE and ribs. 



These changes are of several kinds, and, as a rule, several varieties of modi- 

 fication cooperate in the differentiation of a muscle. Some of the more important 

 are as follow : 



1. An end-to-end fusion of several myomeres or portions of myomeres takes 

 place, producing a muscle-sheet or band which extends uninterruptedly through sev- 

 eral primary segments. Such a modification gives rise to muscles supplied by a 

 number of segmental nerves ; just as many, indeed, as there are myomeres partici- 

 pating in the formation of the muscle. Examples of muscles formed in this way are 

 to be seen in the musculature of the abdominal walls, the oblique muscles, the trans- 

 versalis, and the rectus, for instance, being all polymeric muscles, as are also many 

 of the longitudinal muscles of the back. Not infrequently the origin of these mus- 

 cles by the fusion of portions of successive myomeres is shown, independently of 

 their nerve-supply, by the persistence in their course of some of the intermuscular 

 septa, these forming transverse tendinous bands traversing the muscle in a horizontal 

 direction. Such tendinous inscriptions {inscriptiones tendinece^, as they are termed, 

 occur normally in the rectus abdominis, and' are also frequently found in the internal 

 oblique, the sterno-hyoid, and the sterno-thyroid muscles. 



2. A longitudinal division of the myomeres into a number of distinct and origi- 

 nally parallel portions may occur. Examples of this modification combined with the 

 end-to-end fusion of the portions so formed from successive myomeres are very 

 abundant. Thus, the rectus abdominis is the result of the splitting off of the ventral 

 portion of a number of successive myomeres, whose remaining portions are largely 

 represented in the oblique and transverse abdominal muscles. So, too, in the neck, 

 the differentiation of the sterno-hyoid and omo-hyoid is due to the same process,, 

 and it has also acted in the differentiation of the various muscles of the transverso- 

 costal group of the dorsal musculature. 



3. A tangential splitting of the myomeres is again an occurrence of great fre- 

 quency, producing superposed muscles, and is clearly shown in the dorsal muscula- 

 ture and in the ventro-lateral muscles of the thoracic and abdominal walls. It 

 does not necessarily involve all portions of a myomere when this has already divided 

 longitudinally, but may be confined to only certain of the parts so formed. Thus, 

 while it affects the ventro-lateral abdominal muscles, it does not affect the rectus 

 abdominis, this muscle representing the entire thickness of the ventral borders of a 

 number of successive myomeres. 



4. Associated with the change just described there is frequently a modification 

 in the direction of the fibres in one or more of the superposed muscles. Primarily 

 the fibres of each myomere have a cephalo-caudal direction, — a condition which is 

 still retained in the rectus abdominis, for instance. In the ventro-lateral abdominal 

 and thoracic muscles, however, the original direction of the fibres has been greatly 

 altered, those of the superficial layer being directed in general downward and inward, 

 those of the middle layer to a considerable extent downward and outward, while 

 those of the deepest layer are directed almost or quite transversely, — that is 

 to say, in a direction which is 90° different from that taken by the fibres of the 

 myomere. 



5. An exceedingly interesting modification is that which results from the migra- 

 tion of some of the myomeres over their successors, so that a muscle formed from 

 certain of the cervical myomeres, for example, may in the adult condition be super- 

 posed upon muscles derived from the thoracic segments. In such cases of migration 



