740 DEVELOl'MENT OF THE HEAD. 



in the course of development, clefts which penetrate the wall of the 

 pharynx ; these clefts, or so-called branchial apertures, running com- 

 pletely through the wall of the pliarynx and the external wall of the 

 body of the embryo (see figs. 537 and 538). 



The auditory pit, or primary depression from the epiblastic surface 

 which forms the rudiment of the labyrinth of the ear, is situated imme- 

 diately above the upper or proximal end of the two first postoral 

 plates, and consequently on a level with the first postoral cleft. And 

 this proximity of position is connected with the intimate relation in 

 which two sets of parts stand to each other : for the part called the 

 first branchial cleft is afterwards converted into the external and middle 

 passages of the ear, (meatus, tympanum, and Eustachian canal), the 

 membrana tympani being at a later period thrown across the passage. 

 It forms thus the tympauo-eustachian cleft or canal. The tympanic bono 

 is of membranous origin and is formed round the first cleft. The external 

 auricle is of integumental origin, and is formed in the second postoral 

 bar posteriorly and externally to the aperture of the first cleft. 



The second postoral cleft is the first true water passage, or the first 

 of those which serves as a gill aperture in branchiate vertebrates, and 

 which may in the lower classes be increased to a greater number. 



Although the description of some of the changes which the several 

 pharyngeal plates or branchial arches undergo in the further process 

 of development, belongs to a different part of the subject, yet it may 

 be useful to describe sliortly the more important of them in this place. 



In the first or mandibular arch a strong cartilaginous bar is formed 

 known as the cartilage of Mcclcel, on the exterior of which, but not 

 in its own substance, throughout a considerable part of its distal length, 

 the lower jaw-bone is afterwards developed. The proximal part next 

 the cranium, which comes later to be connected with the auditory 

 capsule, becomes in mannnalia the malleus, in birds and reptiles chiefiy 

 the OS quadratum. (Seefigures in connection with development of the ear.) 



In the second or hyoid arcli are developed the styloid process, the 

 stylo-hyoid ligament, the lesser or upper coruua of the hyoid bone, the 

 series of parts which connect them with the basis of the skull, being 

 united like the first to the auditory capsule : but the proximal part of 

 this arch would appear also to have the incus formed in it, and to 

 be connected with the stapes and stapedius muscle. 



The third arch' is the thyro-hyoid, and is related to the formation of 

 the lower or great cornua, and the body of the hyoid bone. It corre- 

 sponds with the first true branchial arch of amphibia and fishes, in 

 which animals the clefts and bars behind this arch become more 

 numerous than in the higher vertebrata. 



The fourth arch, which has no special name, but might be called suh- 

 hyoid or cervical, does not seem to form the basis of any particular organ, 

 but is situated exactly at that part of the body which becomes elongated 

 as the neck, — a part Avhich may be considered as absent in the foetus, 

 and the formation of which by a simple process of elongation gives ^""^e 

 to some peculiar features in the anatomy of the parts composing it. 



Relations of Cranial ITerves. — The rudiments of four cranial nerves, 

 besides the optic and auditory afterwards to be referred to, are found at 

 a very early period in connection with the plates now under consideration, 

 and the following is the relation in which, according to Parker, they 

 stand to these plates in all vertebrate animals. These nerves are the 



