THE ORGAN OF HEARING. ,77 



CHAPTER XVIII. 



THE ORGAN OF HEARING. 



THE complicated organ of hearing of man and the higher animals, 

 reduced to its essential factors, consists of two parts, the system of 

 intercommunicating epithelial tubes, certain parts of whose walls are 

 differentiated into special structures for the perception of the sound- 

 waves, and the elaborate conducting apparatus for the transmis- 

 sion, direct and indirect, of the sound-impulses to the perceptive 

 structures. 



THE EXTERNAL EAR. 



The external ear, including the pinna and the external audi- 

 tory canal, possesses a bony or cartilaginous basis over which 

 extend the integument and a layer of subcutaneous tissue. 

 The cartilage is of the yellow, elastic variety, forming a thin, tough, 

 yielding plate, displaying the various depressions and elevations seen 

 on the outside ; the lobule, however, contains no cartilage, but only 

 tough fibrous tissue and fat. 



The skin covering the pinna corresponds with the surrounding 

 integument ; within the auditory canal, however, it presents some 

 change. The skin covering the cartilaginous division of the 

 meatus, together with part of the roof of the bony division, is char- 

 acterized by its thickness, the subcutaneous tissue also constituting 

 a layer of considerable depth, which includes some fat and many 

 bundles of dense fibrous tissue. Fine hairs, with relatively very 

 large sebaceous glands, occur in all parts of this surface, as do also 

 the ceruminous glands, which constitute conspicuous structures 

 and closely correspond to the glands of Moll within the eyelid, being, 

 like them, modified sweat-glands. Their long, narrow ducts during 

 early life open with the sebaceous glands into the hair-follicles, but 

 later acquire independent orifices. The ceruminous glands pos- 

 sess a well-marked basement-membrane, within which lies a single 

 layer of cuboidal epithelial cells, with a thin, longitudinal stratum 

 of non-striped muscle-cells interposed. The secreting cells 

 contain numerous brown particles, but the presence of fat is question- 

 able, the fatty constituents of the cerumen being probably contributed 

 by the adjoining sebaceous glands. The coiled masses of the gland- 

 tubes are situated within the subcutaneous tissue, where they some- 

 times reach as far as the cartilage or the bone. 



