DEVELOPMENT. 



683: 



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 afterwards 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, 

 inclosing the first visceral cleft. 



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



7F 



flL 



FIG. 468. Embryo chick (4th day), viewed as a transparent object, lying on its left side (mag- 

 nified). OH, cerebral hemispheres; FB, fore-brain or vesicle of third ventricle, with Pn, pineal 

 gland projecting from its summit: MB, mid-brain; C 6, cerebellum; IV. V, fourth ventricle; Z,, 

 lens; chs, choroidal slit: Cen.V, auditory vesicle; .s m, superior maxillary process; IF. 2F, etc., first, 

 second, third, and fourth visceral folds; V, fifth nerve, sending one branch (ophthalmic) to the eye, 

 and another to the first visceral arch; VII, seventh nerve, passing to the second visceral arch;. 

 G.Ph, glosso-pharyngeal nerve, passing to the third visceral arch; Pg, pneumogastric nerve, pass- 

 ing towards the fourth visceral arch ; iu, investing mass; ch. notochord; its front end cannot be 

 seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend down- 

 wards, and then terminates in a point; Ht, heart seen through the walls of the chest; MP, muscle 

 plates; TF, wing, showing commencing differentiation of segments, corresponding to arm, forearm, 

 and hand; S S, somatic stalk; Al, allaritois; fiTL, hind-limb, as yet a shapeless bud, showing no 

 differentiation. Beneath it is seen the curved tail. (Foster and Balfour.) 



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

 geal 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 succes- 

 sive branchial clefts. 



The Extremities. 



The extremities are developed in a uniform manner in all vertebrate 

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

 etes of the trunk (see Fig. 468), at points where more or less of an arch 



