966 SPECIAL PHYSIOLOGY. 



sist of two layers, which subsequently blend, and their edges, at first 

 separated below, close in that situation, include the central artery of 

 the retina, and form the anterior part of the optic nerve and the retinal 

 expansion. The anterior part of this secondary vesicle corresponds 

 with the pars ciliaris of the retina, and 'gives origin to no nervous ele- 

 ments. The yellow spot does not appear until after birth. The outer 

 coats of the eyeball, or the sclerotic and cornea, are partly growths of 

 the secondary vesicles, and partly derived from the neighboring cutis. 

 The choroid coat, also derived from the secondary vesicle, is at first 

 adherent to the retina; the iris, growing at a later period from the 

 margin of the choroid, forms an im perforate curtain, the central part 

 of which, or membrana pupillaris, becomes transparent, then gradually 

 loses its vessels, and finally disappears. The capsule of the lens is, 

 for a time, invested by a vascular membrane, supplied by the central 

 artery of the retina, and connected with the pupillary membrane and 

 margin of the iris. The aqueous humor is secreted very late, the parts 

 in front of the lens previously touching each other. For a time, the 

 eyeball is simply covered with the integument,.but this rises up, above 

 and below, into small crescentic folds, which become the future eyelids; 

 these, for a time, cohere at their edges, and then separate. 



The Ear. The auditory sacs are not developed, like the optic 

 vesicles, from the cerebral vesicles, but, like the lens and its capsule, 

 from inversions of the common integument. They commence by a little 

 pit or depression, which afterwards becomes completely shut off from 

 the surface, and, receding, is eventually attached to -the side of the 

 medulla. This primitive auditory sac forms the sac of the labyrinth, 

 with which the auditory nerve, an independent formation from the 

 medulla oblongata, is soon connected. From the sac of the labyrinth 

 are gradually developed the membranous semicircular canals, and the 

 winding cochlea, with the fluids in those cavities. All these parts are 

 at first straight processes, but afterwards become curved or spiral. 

 The cavity of the tympanum, with its ossicles, the tympanic bone, and 

 the auricle, are formed externally to the deeper parts, in connection 

 with the pharynx and Eustachian tube, as will be described with the 

 development of the face. The osseous walls, which afterwards invest 

 the labyrinth, are formed from the primitive cartilage of the base of 

 the cranium. The mastoid process is not hollowed out into large air- 

 cells until after puberty. The external meatus and the auricle are 

 outgrowths of the annular fibro-cartilage, which forms the tympanic 

 bone. The auditory gray nervous centre arises, near the posterior 

 pyramid and restiform body of the medulla oblongata, as two masses, 

 the outer one of which gradually moves backwards into the cerebellum 

 itself. The auditory nerve consists of two portions, both of which be- 

 come connected with these masses ; but the anterior portion of the 

 nerve also joins the superior peduncle of the cerebellum, and even 

 reaches the inferior vermiform process. (L. Clarke.) 



