682 HANDBOOK OF PHYSIOLOGY. 



than the others, and of these the second is the most conspicuous. The 

 first fold nearly meets its fellow in the middle line, the second less 

 nearly, and the others in order still less so. Thus in the neck there is 

 a triangular interval, into which by the splitting of the mesoblast at 

 that part the pleuro-peritoneal cavity extends. The branchial clefts 

 and arches are not all permanent. The first arch gives off a branch 

 from its front edge, which passes forwards to meet its fellow, but these 

 offshoots do not quite meet, being separated by a process which grows 

 downwards from the head. Between the branches and the main first 

 fold is the cavity of the mouth. The branches represent the superior 

 maxilla, and the main folds the mandible or lower jaw. The central 

 process, which grows down, is the fronto-nasal process. 



In this way, the so-called visceral arches and clefts are formed, four 

 on each side (Fig. 467, A). 



From or in connection with these arches the following parts are de- 

 veloped: 



FIG. 467. 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 is 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 process; 6, the superior 

 maxillary process; 7, the lower jaw; x, the tongue; 8, first branchial cleft becoming the meatus 

 auditorius externus. 



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

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

 while the malleus is ossified from the proximal end. 



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 

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

 two deformities frequently co-exist, they are by no means always neces- 

 sarily associated. 



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

 so-called frontal-nasal process (A, 3, Fig. 467). From the second arch 

 are developed the incus, stapes, and stapedius muscle, the styloid pro- 

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

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



