THE MUSCULAR SYSTEM 



213 



mass into the separate muscles of the adult limb is mostly completed by 

 the end of the second month. 



Visceral Muscles, Derived from the Hypomere. The visceral or 

 hypomeric muscles include those of the heart and main blood vessels as 

 well as those associated with the alimentary canal. While most of them 

 consist of smooth muscle fibers, the visceral muscles of the head and heart 

 are striped. 



The muscles of the wall of the alimentary canal are formed from 

 mesenchymatous cells proliferated from the visceral layer of the hypomere. 

 Such cells fill the space between the mucous epithehum lining the aU- 



miwmmniiimimmrj ffi 



•/MYOTOMES 1-4 



1ST. CERVICAL 

 MYOTOME 



ANL M TRAPEZIUS* 

 "M. STERNOCLEIDO- 

 MASTOID 



^SPINAL 

 /GANGLIA 



MANDIBULAR MUSCLES' 



ANLACE DIAPHRAGM 



1ST. THORACIC MYOTOME' _ 



Fig. 201. — The anlagen of the cranial muscles with their nerve relations as seen in a 

 7 mm. human embryo. (Redrawn from Keibel and Mall, after W. H. Lewis.) 



mentary canal and the adjacent hypomere. They also differentiate into 

 both the connective tissues and the blood-vessels of the wall of the ali- 

 mentary canal and into its circular and longitudinal muscles. The 

 circular layer of muscles is formed before the longitudinal layer. 



The fate of the hv-pomere in the head is much more complex than in 

 the trunk. Besides forming the heart and pericardium, the head hypo- 

 mere gives rise to the chewing muscles, the muscles of expression, and the 

 pharyngeal and laryngeal muscles. In general, the processes involved 

 are similar in lower and higher vertebrates. 



In embryos of lower vertebrates, e.g., elasmobranchs, the coelom 

 extends throughout head and trunk. In the head region, as a result of 

 the outpocketing of pharyngeal pouches, the h^-pomere becomes divided 

 into a series of pouches each of which lies in a visceral arch. This hypo- 

 meric segmentation (branchiomerism) is independent of the segmentation 



