THE NECK 149 



which gives rise to the mouth and pharynx. There are six of 

 these arches, but the lower two do not cause any elevation on the 

 ectodermal surface. The arches are separated from one another 

 on the outside by grooves which are termed the visceral clefts, 

 and on the inside by grooves which are termed the visceral 

 pouches. The grooves, which never break down in the human 

 embryo, consist of an outer layer of ectoderm and an inner layer 

 of entoderm, the two together forming the cleft membrane. 

 Each arch contains a bar of cartilage, a muscle mass, a nerve, 

 and an artery. 



At the period when the branchial arches develop, the embryo 

 possesses two ventral aortse, which arise from the truncus 

 arteriosus, and two dorsal aortas. Each ventral aorta is con- 

 nected with the dorsal aorta of the same side by six aortic arches, 

 each of which supplies one branchial arch and the cleft caudal 

 to it. The external carotid is derived from the distal portion 

 of the ventral aorta, while the internal carotid represents 

 the third aortic arch and the distal part of the dorsal aorta. 

 From this it is clear that branchial fistulse, which develop in 

 connection with the first or second clefts, must pass deep to 

 the external carotid but superficial to the internal carotid 

 artery. 



The cartilage of the first arch is known as Meckel's cartilage. 

 It is almost entirely replaced by the mandible, but its cephalic 

 extremity forms the greater part of the malleus. The cartilage 

 of the second arch forms the tip of the styloid process, the stylo- 

 hyoid ligament, the lesser cornu, and a part of the body of the 

 hyoid bone. The cartilage of the third arch forms the greater 

 cornu and the remainder of the body of the hyoid bone, while 

 the cartilages of the lower arches take part in the formation of 

 the framework of the larynx. 



The muscle mass of the first arch develops into the muscles of 

 mastication, which are supplied by the nerve of the first arch, 

 the mandibular division of the trigeminal. The platysma and 

 the muscles of facial expression are derived from the muscle mass 

 of the second arch, and are innervated by the facial nerve, which 

 is the nerve of the second arch. The muscle mass of the third 

 arch forms the stylo-pharyngeus , and its nerve-supply is obtained 

 from the glosso-pharyngeal, which is the nerve of the third arch. 

 It is probable that the constrictor muscles of the pharynx have 

 a similar origin. Lower down in the series our knowledge of 

 the history of the muscle mass is less definite. The crico-thyreoid 



