. The comparative anatomy of the domesticated animals. Veterinary anatomy. 910 EMBRYOLOGY. Fig. is gradually developed, and at last become a closed cavity. At this time, the wall of the labyrinth is only a simple epithelial membrane ; this is soon covered outwardly, by a vascular connective membrane that separates into three layers : an internal, joined to the epithelium to form the membranous labyrinth; an external, that lines the cartilaginous labyrinth; and a middle whose soft, embryonic, connective tissue disappears, and is replaced by the' perilymph. At the same time that these chan
. The comparative anatomy of the domesticated animals. Veterinary anatomy. 910 EMBRYOLOGY. Fig. is gradually developed, and at last become a closed cavity. At this time, the wall of the labyrinth is only a simple epithelial membrane ; this is soon covered outwardly, by a vascular connective membrane that separates into three layers : an internal, joined to the epithelium to form the membranous labyrinth; an external, that lines the cartilaginous labyrinth; and a middle whose soft, embryonic, connective tissue disappears, and is replaced by the' perilymph. At the same time that these changes of structure are occurring, the vesicular form of the labyrinth is modified, and shows the cochlea, semicircular canals, utriculus, and sacculus. The middle and external ear arise from the first pharyngeal (branchial) cleft, which is never completely closed, while the others disappear. At first,' then, we find a cavity communicating externally with the pharynx; this cavity contracts, and is separated into two portions by a' partition that spreads across its middle, and which becomes the membrana tympani; the internal portion is the middle ear and Eustachian tube; the external portion is the external auditory canal. The ossicula auditus appear in a cartila- ginous state towards the third month ; they gradually ossify, and have scarcely acquired their definitive volume at birth. The ex- ternal ear (concha) is developed above the integument after the second month, 3. Olfactory Apparatus.—This com- mences by two depressions of the epidermic lamina, analogous to the auditory fossa and that of the lens. These two olfactory fosssB appear below the ocular vesicles, and become more and more distinct, being margined by small projections which increase their depth. Behind, they communicate with the pharynx. The appearance of the palate separates them in front from the buccal cavity, and from this period the nasal fosste are constituted. They are completed by the development of the b
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