THE MUSCULAR SYSTEM 



291 



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

 meric segmentation (branchiomerism) is independent of the segmentation 

 of the epimere (mesomerism), and should not be confused with this, 

 although it is possible that the two types of segmentation may originally 

 have coincided. From the mesoderm of the visceral arches arise the 

 muscles, connective tissues, and blood vessels of the arches. In the 

 fishes, these muscles are differentiated into levators, depressors, and 

 constrictors of the gills. In the process of conversion the epithelium of 

 the hypomere breaks up into mesenchyme and the coelomic cavity 

 disappears. 



.^MYOTOMES 1-4 



ANL. M. TRAPEZIUS + 

 "M. STERNOCLEIDO- 

 MASTOID 



MANDIBULAR MUSCLES/ ■-•^--^^-^ ^^C^ ^::^^ ^'ANLACE DIAPHRAGM 



1ST THORACIC MYOTOME''' ^^ 



Fig. 243. — The anlagen of the cranial muscles with their nerve relations as seen in a 

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



In mammals and man, the coelom is absent in the visceral arches and 

 the muscles are formed from masses of mesenchymatous cells. From 

 the first visceral arch arise the muscles innervated by the mandibular 

 branch of the fifth nerve, the masseter, temporalis, pterygoid, mylohyoid, 

 and tensor veli palatini. From the same source come the tensor tympani 

 of the ear and the anterior belly of the digastricus. The muscles inner- 

 vated by the facial nerve are derived from the second visceral arch, the 

 hyoid. They include the muscles of expression, the stylo-hyoid, stapedius, 

 and the posterior belly of the digastricus. From the third visceral arch 

 arise the stylopharyngeus muscle innervated by the glossopharyngeus 

 nerve, and the constrictors of the pharynx innervated by the vagus nerve. 

 The laryngeal muscles innervated by the vago-accessory nerve originate 

 from the fourth and fifth visceral arches. As already explained the 

 muscles of the tongue and throat innervated by the hypoglossal nerve 

 are myotomic, not visceral, in origin. 



