DEVELOPMENT OF THE SOMITES 17 



and sections will show that similar and corresponding grooves also occur upon the inner surface 

 of the pharyngeal wall. These are the branchial grooves, and since they are four in number 

 (with a rudimentary fifth) in the human embryo, they mark off five branchial (or visceral) arches, 

 the first of which hes between the oral depression and the first branchial groove, while the fifth 

 is situated behind the fourth groove. These branchial arches are so named because they repre- 

 sent the arches which (excepting the first) support the gills (branchiae) in the lower vertebrates, 

 the grooves representing the branchial slits, even although they do not become perforated in 

 the human embryo. 



Each branchiomere consists of an axial skeletal structure, of muscles which act on this skel- 

 eton, of a nerve which supplies the muscles and the neighbouring integument and mucous mem- 

 brane, and of an artery which carries blood to all these structures. The arches, however, do 

 not in the human embryo retain their original branchial function, but undergo extensive modi- 

 fications, becoming adapted to various functions and showing less in the adult of their originally 

 simple arrangement than do the metameres. Nevertheless no matter what modifications the 

 musculature of any arch may undergo, it will retain its original innervation and, to a large 

 extent, its relations to the skeletal elements of its arch; and even the arteries in their distribu- 

 tion show clear indications of being arranged in correspondence to the various arches. 



Fig. 21. — Diagram of a Transverse Section through the Thoracic Region. 

 (The pleura is represented in blue and the pericardium in red.) 



With respect to the fate of the various structures pertaining to each branchial arch, their 

 general arrangement in the adult body may be stated in the following table: — 



Relations of the Branchial Arches in the Adult 



Muscles 



Nerve 



First arch Mandible, malleus and 



incus. 

 Second arch Hyoid (lesser cornu), 



styloid process and 

 I stapes. 

 Third arch Hyoid (greater cornu) . . 



Masticatory, mylohoid and di- 

 gastric (ant.), tensor tympani. 



Stylohyoid, digastric (post.), 

 muscles of expression, stape- 

 dius. 



Pharyngeal 



Trigeminus. 

 Facialis. 



Glossopharyn- 

 geus. 



Fourth and fifth Thyreoid cartilage Pharyngeal and laryngeal Vagus. 



arches. I I 



Branchial grooves. — Of the external branchial grooves, the first (lying between mandibular 

 and hyoid arches) becomes deepened to form the external auditory meatus, the margins becom- 

 ing elevated to form the auricle (fig. 26). The region corresponding to the second, third and 

 fourth external grooves becomes depressed, forming the sinus cervicalis, which soon closes up 

 and disappears. 



The internal branchial grooves or pouches communicate with the pharyngeal cavity and are 



