214 



CHORDATE ANATOMY 



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. 



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 veH palatini. (Fig. 201) From the same source come the 

 tensor tympani of the ear and the anterior belly of the digastricus. The 

 muscles innervated 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 

 glossopharyngeal 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 

 ah-eady explained, the muscles of the tongue and throat innervated by the 

 hypoglossal nerve are myotomic, not visceral, in origin. 



