794 



HANDBOOK OF PHYSIOLOGY. 



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 hy old bone. From the third visceral arch, the greater cornu and body 

 of the hyoid bone. In man and other mammalia t he 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 glcsso-pharyngeal, 

 and the vagus. The ophthalmic division of the trigeminal supplies the 

 fronto-nasal process; 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, inclosing the first visceral cleft. 



Similarly, the glosso-pharyngeal divides to inclose the second visceral 

 cleft, its lingual branch being distributed to the second, and its 

 pharyngeal branch to the third arch. 



The vagus, too, sends a branch (pharyngeal) along the third arch, 

 and in fishes it gives off paired branches, which divide to inclose several 

 successive branchial clefts. 



The Extremities. 



The extremities are developed in a uniform manner in all verte- 

 brate animals. They appear in the form of leaf -like elevations from the 



Fig. 488. A human embryo of the fourth week, 3^ lines in length. 1, the chorion; 3, part 

 of the amnion; 4, umbilical vesicle with its long pedicle passing into the abdomen; 7, th*. 

 heart ; 8, the liver ; 9, the visceral arch destined to form the lower jaw, beneath which are two 

 other visceral arches separated by the branchial clefts; 10, rudiment of the upper extremity; 11, 

 that of the lower extremity; 12, the umbilical cord; 15, the eye; 16, the ear; 17, cerebral hemi- 

 spheres; 18, optic lobes, corpora quadrigemina. (Miiller.) 



parieties of the trunk (see fig. 488), at points where more or less of an 

 arch will be produced for them within. The primitive form of the 

 extremity is nearly the same in all vertebrata, whether it be destined for 



