EAR 481 



The external acoustic meatus is lined with skin continuous with the 

 cutaneous layer of the tympanic membrane. In the deep or osseous por- 

 tion the skin is very thin, without hair or glands except along its upper 

 wall. There and in the outer or cartilaginous part, ceruminous glands are 

 abundant. "They are branched tubulo-alveolar glands" (Huber) which 

 in many respects resemble large sweat glands. Their ducts are lined with 

 stratified epithelium. The coils consist of a single layer of secreting cells, 

 general cuboidal, surrounded by smooth muscle fibers and a well-defined 

 basement membrane. They differ from sweat glands in that their coils 

 have a very large lumen, especially in the adult; and their gland cells, 

 often with a distinct cuticular border, contain many pigment granules and 

 fat droplets. Their narrow ducts in adults end on the surface of the skin 

 close beside the hair sheaths; in children they empty into the sheaths 

 (Fig. 485). It has not been shown that the ceruminous glands are more 

 directly concerned in the production of cerumen than the sebaceous 

 glands. The cerumen obviously is an oily rather than a watery secre- 

 tion, and it contains fatty cells and pigment. 



The cartilage of the external acoustic meatus and of the auricle is 

 elastic. 



NOSE. 



The nasal cavities are formed by the invagination of a pair of epi- 

 dermal thickenings similar to those which give rise to the lens and auditory 

 vesicle. The pockets thus produced in the embryo are called "nasal pits" 

 (Fig. 205, n, p. 216). Their external openings 

 remain as the nares of the adult, but tempor- 

 arily, from the third to the fifth month of em- 

 bryonic life, they are closed by an epithelial 

 proliferation. Each nasal pit acquires an in- 

 ternal opening, the choana, in the roof of the 

 pharynx. The choanae are at first situated 

 near the front of the mouth, separated from 

 one another by a broad nasal septum (Fig. 487). 

 As the latter extends posteriorly, it is joined by 



FIG. 487. THE ROOF OF THB 



the Palate processes which grow toward it from MOUTHOF A HUMAN EMBRYO 



OF 8 WEEKS. X 4. (After 



the sides of the maxillae. Thus the choanae re- Koiimann.) 



na, Nans; ch., choana; al. p., i. p., 



Cede tOWard the back Of the mOUth While and pa. p., alveolar, intermax- 



illary, and palate processes. 



the embryonic condition of cleft palate is being 



removed (Fig. 488). The lateral walls of the nasal cavities produce three 

 curved folds one above another; they are concave below, and in them 

 the concha (turbinate bones) develop. The nasal mucosa covers these 

 and extends into excavations in the adjacent bones, forming the sphenoid, 

 31 



