100 OBSTETRICAL PHYSIOLOGY. 



changes of structure are taking place, the vesicular shape of the laby- 

 rinth is modified, and shows the cochlea, semicircular canals, utricuhis, 

 and saccuhis. The middle and external ear are formed by the first 

 pharyngeal slit, which is never completely closed, while the others dis- 

 appear. 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 ^y??z_/;a?z?r?H, while the 

 inner cavity forms the middle car and Eustachian tube, and the external 

 portion the external auditory canal or meatus. The ossicula auditus are 

 at first cartilaginous, and appear towards the third month ; after which 

 thev gradually ossify, and have nearly assumed their definitive shape at 

 birth. 



The concha is developed beneath the integument after the second 

 month. 



The organs of smell begin by two depressions in the epidermic layer, 

 analogous to the crystalline lens and auditory fossettes. These two 

 olfactory fossa appear below the ocular vesicles, and, becoming deeper, 

 their depth is further increased by granulations which spring up on 

 their borders. 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 nostrils are formed by a collection of mucus 

 and epithelium ; they open towards the middle period of gestation. 



T]ie Skin and its Appendages. 



The skin and its appendages, which might be designated the tactile 

 apparatus, are developed by the epidermic and middle layer of the 

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

 ments, form the derm, in which vessels are readily seen after three 

 months. In the epiderm it is not long before the mucous and horny 

 layers can be distinguished ; in the first, pigment cells are observed at 

 the commencement of the fifth month in the lai'ger quadrupeds. The 

 epiderm is easily detached from the derm ; it forms a peculiar white, 

 completely-enveloping pellicle on the surface of the latter, apparently 

 separated from it by the growing hairs. Frequently we find the integu- 

 ment covered by a special coating that looks like varnish (the varnix 

 caseosa) ; this would appear to be intended to protect the epidermic 

 epithelium from the destructive solvent action of the alkaline amniotic 

 fluid. It is very abundant on the human foetus at all periods, but is 

 not found on that of animals so long as the skin is glabrous. As soon 

 as the hairs begin to appear, the epiderm is partially detached in the 

 form of the thin pellicle just alluded to, and the decomposition of which 

 gives rise to an appearance like varnish. It is best seen in the foetal 

 Pig, the hairs on the skin appearing all at once over the body ; in the 

 other domesticated animals they are only developed successively, and 

 consequently the shedding of the epiderm occurs partially and in 

 patches, which are insensibly confounded with the normal epidermic 

 layers. Microscopically, these flakes offer the same characters as 

 epidermis removed by a vesicant ; the points where the hairs have 

 passed appear as regular infundibuliform openings. When the fcEtus 

 increases in volume the epiderm desquamates, and the debris floats in 

 the amniotic fluid. 



