246 MUSCLES 



The cranial extension of the pocket disappears, but when the stomach turns on its 

 axis the diverticulum enlarges horizontally between liver and stomach, and is 

 bounded on the left by the displaced mesogaster. The mesogaster is connected, 

 with the greater curvature i.e. the original dorsal border of the organ and is 

 thus deflected to the left. It now grows down as a double fold, containing an 

 extension of the lesser sac, to form the greater omentum. This passes down at first 

 to the left of the colonic loop, but when the definitive positions are assumed it 

 comes to lie in front of it. An adhesion then takes place between the posterior 

 layers and the mesocolon, so that the colon comes to be attached to the omentum. 

 Further, by a disappearance of the posterior layer of the double mesogaster, the 

 pancreas, which at first lies between its lamellae, comes to lie behind the peritoneum, 

 just above the line along which the mesogaster is joined to the mesocolon (fig. 302). 

 The duodenum also loses its mesentery, either by fusion of the visceral and parietal 

 layers of peritoneum, as some describe it, or by a stripping off of the peritoneum from 

 its posterior aspect, due to the covering layer not expanding proportionately to the 

 gut-wall. 



DEVELOPMENT OF THE MUSCULAK SYSTEM. 1 



Muscles of the trunk. We have already studied the early phases in the 

 development of the myo tomes (p. 56). In the human embryo during the third 

 week the muscle-plate is produced from the inner wall of the mesodermic segment ;. 

 the axial mesenchyme is formed from the sclerotomes ; the mesenchyme of the 

 somatopleure becomes a thick layer, and the mesenchymatous thickenings on 

 the Wolffian ridges which constitute the rudiments of the limb-buds are laid down. 

 During the fourth week the myotomes become greatly lengthened in the trunk and 

 extend into the somatopleure. Their inner wall has become entirely converted 

 into muscle-cells, but the outer wall is still epithelial, and the cavity (myoccel) 

 has become obliterated by the fusion of the two lamellae (cf. fig. 84, p. 59). 

 During the fifth week the outer wall also becomes muscular (Bardeen and 

 Lewis ;,* and the myotomes are joined into a dorso-ventral sheet in which 

 the original segmentation has largely disappeared. This sheet next becomes 

 subdivided by ingrowing mesenchyme septa into a dorsal and a ventro-lateral 

 mass. During the sixth week the dorsal begins to be separated from the ventro- 

 lateral mass ; the dorsal section becomes subdivided into three longitudinal 

 columns (ilio-costalis, longissimus dorsi, spinalis dor si] ; and the ventral-lateral into 

 mesial and lateral portions, of which the mesial forms the rectus, and the lateral, 

 cleaving into three strata, the obliquus externus and internus and transversalis. 

 It is in the thorax alone that any segmental arrangement is retained, the ventral 

 projections of the myotomes being separated by the rudiments of the ribs and 

 giving origin to the intercostal muscles. By the seventh week the premuscular 

 tissue is all resolved into its definitive divisions (Bardeen and Lewis). 



The rectus-muscle rudiments are at first separated by a considerable interval 

 (Mall), the body-wall between them being formed only by the membrana reunions 

 or fused somatopleuric layers. The muscle-sheets ultimately grow towards the 

 middle line, and come into apposition, carrying their nerves with them. 



Muscles of the limb?. In the early stages the limb -buds consist of a mass 

 of mesenchyme continuous with that of the Wolffian ridges and lateral to the line 

 of the myotomes (fig. 82, p. 57). As the buds increase in length this becomes 

 differentiated into a skeletal core and a premuscule sheath. It is not definitely 



1 The literature of the development of the muscles will be found collected by Maurer in Hertwig's 

 Handbuch III. Th. i. p. 76 seq. ; also in Kollmann's Handatlas, Appendix, p. 45 seq. 

 - Bardeen and Lewis, Amer. Jour, of Anat. i. 



