92 



OBSTETRICAL PHYSIOLOGY. 



The protective and motor apparatus of the eye are gradually developed 

 around the globe. The eyelids are small cutaneous folds which are 

 formed in the first third of uterine life, and grow and unite by their mar- 

 gins until a short time before or after birth, according to the species, 

 when they separate. They are maintained closed by a thin membrane, 

 which disappears in solipeds, ruminants, and the Pig, before birth ; but in 

 carnivora it remains until some days (eight or ten) after that event. So 

 long as the lids are closed, the conjunctiva is only a sac communicating 

 with the lachrymal canal. The crystalline lens in the foetus and new- 

 born animal distinctly shows the three septa peculiar to it ; three diverge 

 from each pole at angles of 120°. The lachrymal gland is an appendage 

 of the epithelial layer which is intruded above the globe ; at first com- 

 pact, it becomes gradually excavated into cavities, from which arise the 

 excretory ducts. 



The auditory apparatus, consisting of the internal ear, auditory nerve, 

 and middle ear, is developed separately. The labyrinth appears in the 

 form of a vesicle which is not in direct relation with the posterior cere- 

 bral cell, but is constituted by a depression of the epidermic layer — the 

 auditory fossa — that becomes more and more marked until it is finally a 

 closed cavity. At this time the wall of the labyrinth is only a simple 

 epithelial membrane ; but this is soon increased externally by a connec- 

 tive membrane which vascularizes it, and then gives rise to three layers ; 

 an internal, which adheres to the epithelium to form the membrane of 

 the labyrinth ; an external, that lines the labyrinthic cartilage ; and a 

 middle, whose soft embryonic connective tissue disappears and is re- 

 placed by a fluid, the perilymph. At the same time that these changes 

 of structure are taking place, the vesicular shape of the labyrinth is mod- 

 ified, and shows the cochlea, semicircular canals, utriculus, and sacculus. 

 The middle and external e^r are formed by the first pharyngeal slit, which 

 is never completely closed, while the others disappear. At first there is 

 a cavity communicating externally by the pharynx ; this cavity contracts, 

 then divides into two portions by a septum in its middle ; this septum 

 becomes the tynipa?ium, while the inner cavity forms the middle ear and 

 Eustachian tube ; and the external portion the external auditory canal ox 

 meatus. The ossicula audittis are at first cartilaginous, and appear to- 

 wards the third month ; after which they gradually ossify, and have nearly 

 assumed their definitive shape at birth. 



The co?icha is developed beneath the integument after the second 

 month. 



The 07'gans of sinell begin by two depressions in the epidermic layer, 

 analogous to the crystalline lens and auditory fossettes. These two olfac- 

 tory fossa appear below the ocular vesicles, and, becoming deeper, their 

 depth is further increased by granulations which spring up on their bor- 

 ders. Behind, they communicate with the pharynx, and the formation of 

 the palate separates them in front from the buccal cavity. From this time 

 the nasal fossae are constituted and completed by the development of the 

 bones of the face. The olfactory lobes and nerves are at first tubular, and 

 are related to the anterior cerebral vesicle. In the young foetus the nos- 

 trils are formed by a collection of mucus and epithelium ; they open 

 towards the middle period of gestation. 



The Skill and its Appendages. 

 The skiji and its appendages, which might be designated the tactcle ap- 



