THE ORGAN OF HEARING. 383 



tissue vary greatly with age ; in early childhood it is present almost 

 in all parts of the tube, but in adolescence it is plentiful only in the 

 lower third, in the upper third being entirely wanting and in the 

 middle third very sparingly distributed. Small mucous glands 

 are also present, and open on the surface of the mucosa within the 

 depressions between the longitudinal folds ; these glands may exist 

 throughout the length of the tube, but they occur with constancy 

 only towards its pharyngeal end. 



The submucous layer is well developed in the cartilaginous 

 division of the tube, particularly in the outer membranous wall ; it 

 consists of loosely-arranged fibro-elastic tissue, whhch supports the 

 mucous glands and the larger vessels and nerves, and often contains 

 a considerable mass of fat. 



The blood-vessels supplying the tubal mucous membranes are 

 derived from the pharynx and from the tympanum ; the larger longi- 

 tudinal stems run within the submucosa or the deep periosteal layers 

 and send twigs into the mucosa to form capillary net-works. 



The nerves derived from the pharyngeal and tympanic plexuses 

 occupy the deeper layers of the mucosa ; the twigs given off from 

 the larger trunks form a plexus within the superficial parts of the 

 tunica propria, fine non-medullated fibrillae passing to the epithelial 

 structures ; ganglion nerve-cells are found at the nodal points within 

 the plexus. 



THE INTERNAL EAR. 



The internal ear in its fully-developed condition consists of two 

 concentrically arranged parts, the bony and the membranous 

 labyrinth, separated by an intervening space containing the peri- 

 lymph. 



THE SACCULE AND THE UTRICLE. 



While the bony labyrinth in the cochlea and the semicircular 

 canals quite closely repeats the general arrangement of the corre- 

 sponding parts of the enclosed membranous tube, the central divis- 

 ion of the osseous capsule, the bony vestibule, differs somewhat 

 in its details from the enclosed membranous compartments. 



These are two almost completely separated vesicles of un- 

 equal size, the anterior and smaller sacculus and the posterior and 

 larger utriculus ; the compartments communicate indirectly with 

 each other by means of the ductus endolymphaticus, while the 

 saccule connects additionally with the cochlear division of the 

 membranous labyrinth through the narrow canalis reuniens, the 

 utricle directly opening into the semicircular canals. 



The bony wall of the vestibule is lined by a very thin perios- 

 teum, composed of a felt-work of resistant fibrous tissue, in which 

 pigmented connective-tissue cells are not infrequent. From 



