910 



EMBRYOLOGY. 



Fig. 436. 



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 bones of the face. 



The olfactory bulbs and nerves are primarily hollow, and joined to the 

 anterior cerebral cell. The nostrils are formed, in the young foetus, by a 

 mass of mucus and epithelium ; towards the middle of gestation they are 

 open. 



i. GusTATOEY Appaeatus. — /See, subsequently, the development of the 

 tongue in digestive apparatus. 



5. Tactile Apparatus. — The sTcin and its dependencies. — The skin is 

 developed at the expense of the epidermic lamina of the middle layer of 

 the blastoderm. The cutaneous laminae, by the modification of their ele- 

 ments, form the derma, in which the blood-vessels are very apparent at the 

 third month. In the epidermis, the mucous and the horny layers are soon 

 distinguished ; in the first, the pigment is visible at the commencement of 

 the fifth month in the larger quadrupeds. When the foetus increases in 

 volume, the epidermis exfoliates and its debris floats in the liquor amnii. 



In the third month, the hairs are seen in the foetus of the Mare and Cow ; 

 they appear at first on the eyebrows, lips, and the joints of the limbs ; at 



AS E5IBRY0 (HUJIAS) OF FOUR 

 WEEKS, ENLARGED ABOUT THREE 

 TIMES. 



a. 



Vesicle of corpora quadrigemina ; 



b, Vesicle of cerebral hemisplieres ; 



c, Vesicle of tliird ventricle ; d. 

 Vesicle for cerebellum and medulla 

 oblongata ; e. Auditory vesicle ; /, 

 Olfactory fossa ; A, Liver ; * *, 

 Caudal extremity. 



