274 HAND-BOOK OF PHYSIOLOGY. 



The first arch (mandibular) contains a cartilaginous rod (Meckel's 

 cartilage), around the distal end of which the lower jaw is developed, 

 while the malleus is ossified from the proximal end. 



From near the root of this arch the maxillary process grows forward 

 and inward toward the middle line; from it are formed the superior max- 

 illary and malar bones. A pair of cartilaginous rods (ptery go-palatine), 

 parallel to the trabeculae cranii, give origin to the external pterygoid plate 

 of the sphenoid and the palate bones. 



The cleft between the maxillary process and the mandibular (or first 

 visceral arch) forms the mouth. 



When the maxillary processes on the two sides fail partially or com- 

 pletely to unite in the middle line, the well-known condition termed cleft 

 palate results. When the integument of the face presents a similar defi- 



FIG. 430. A. Magnified view from before of the head and neck of a human embryo of about three 

 weeks (from Ecker). 1, anterior cerebral vesicle or cerebrum; 2, middle ditto; 3, middle or fronto- 

 nasal process : 4, superior maxillary process; 5, eye; 6, inferior maxillary process, or first visceral 

 arch, and below it the first cleft; 7, 8, 9, second, third, and fourth arches and clefts. B. Anterior view 

 of the head of a human foetus of about the fifth week (from Ecker, as before, Fig. IV.). 1, 2, 3, 5, the 

 same parts as in A; 4, the external nasal or lateral frontal pi-ocess; 6, the superior maxillary pro- 

 cess; 7, the lower jaw; x, the tongue; 8, first branchial cleft becoming the meatus auditorius ex- 



ternus. 



ciency, we have the deformity known as hare-lip. Though these two 

 deformities frequently co-exist^ they are by no means always necessarily 

 associated. 



\ The upper part of the face in the middle line is developed from the 

 so-called frontal-nasal process (A, 3, Fig. 430). From the second arch 

 are developed the incus, stapes, and stapedius muscle, the styloid process 

 of the temporal bone, the stylo-hyoid ligament, and the smaller cornu of 

 the hyoid bone. From the third visceral arch, the greater cornu and 

 body of the hyoid bone. In man and other mammalia the fourth visceral 

 arch is indistinct. It occupies, the position where the neck is afterward 

 developed. 



A distinct connection is traceable between these visceral arches and 

 certain cranial nerves: the trigeminal, the facial, the glosso-pharyngeal, 

 and the pneumogastric. The ophthalmic division of the trigeminal sup- 

 plies the trabecular arch; the superior and inferior maxillary divisions 

 supply the maxillary and mandibular arches respectively. 



The facial nerve distributes one branch (chorda tympani) to the first 

 visceral arch, and others to the second visceral arch. Thus it divides, 

 enclosing the first visceral cleft. 



