THE VISCERAL AKCIIES AND CLEFTS. 



7 6l 



length, coalesce, and complete by their union the spinal 

 canal. The same process essentially takes place in the 

 head, so as to enclose the cranial cavity. 



The so-called visceral lamina have been also described 

 as passing forwards, and gradually coalescing in front, as 

 the dorsal laminae do behind, and thus enclosing the 

 thoracic and abdominal cavity. An 

 analogous process occurs in the facial 

 and cervical regions, but the enclosing 

 laminae, instead of being simple, as in 

 the former instances, are cleft. 



In this way the so-called visceral arches 

 and clefts are formed, four on each side 

 (fig. 230 A), and from or in connection 

 with these arches the following parts are 

 developed : 



From the first arch, and its m axillary- 

 process, the superior maxillary, the palate 

 bone, and the internal pterygoid plate of 

 the sphenoid bone, the incus and malleus 

 and the lower jaw. The upper part of the 

 face in the middle line is developed from 

 the so-called fronto-nasal process (A, 3, 

 fig. 230). From the second arch are de- 

 veloped the stapes, the stapedius muscle, 

 the styloid process of the temporal bone, 



* Fig. 230 A. Magnified view from before of the head and neck of a 

 human embryo of about three weeks (from Ecker) I, 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.). 

 J 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 ; +, 

 the tongue ; 8, first branchial cleft becoming the meatus auditorius 

 externus. 



