ORGANS OF SENSE. 153 



h — ^, second branch. Figs. 41 and 12, two sections of the nasal caVities : 

 fig. 41, the lower, fig. 42, the upper : a, outline of the nose ; 5, c, nasal 

 bones ; d, e, frontal bone ; J] g, h, t, ethmoid bones ; g, i, k, Z,.alar process and 

 sphenoid bone (cut edge); /•, r, middle lamina of the ethmoid bone (cut edge); 

 s, s, and t, t, thickness of the raucous membrane of the nose ; u, u, length and 

 breadth of the left, v, v, of the right nasal cavity ; iv, w^ x, x, cells of the 

 ethmoid bone ; y, 2, membrane reflexed. Fig. 43, posterior surface of a 

 vertical section of the nose ; a, b, nasal cartilages ; c, c, septal cartilage ; 

 d, d, Schneiderian membrane ; e, e, hairs and mucous glands ; f, g, nostrils. 

 Fig. 44, posterior sectional surface of the nasal cavities : a — -/, cut edge of 

 various bones ; g, h, lachrymal sac and lachrymal duct opening into the 

 inferior meatus ; i, Ic, (, m, section of the Schneiderian membrane ; n, ?i, the 

 surface of the membrane turned towards the nasal cavity ; p, q, lachrymal 

 sac and lachrymal duct of the left side ; r, r, inner bony wall of the maxil- 

 lar}^ sinus. Fig. 45, posterior sectional surface of the nasal cavities and 

 the communicating sinuses : a, i, frontal bone ; c — I, ethmoidal ; m, n, left 

 and right superior turbinated bones ; p, p, vomer ; q — ?/, upper jaw ; a, f, 

 Schneiderian membrane : g, g, opening of the left lachrymal canal ; h, open- 

 ing of the right ; k, 1, m, opening of the maxillary sinus on the right side, 

 m is a probe introduced ; n, o, p, section of the periosteum of the orbit ; s, 

 section of the soft palate ; '~% teeth. 



3. j^NATOilY OF THE Ear, OR OrGAN OF HEARING 



By means of the sense of hearing an animal is enabled to take cognisance 

 of sounds and of all their variations. Sound is the result of any impulse 

 conveyed to the organs of hearing, by undulations of air, these organs being 

 so constructed as to receive these undulations, and so organized as to become 

 sensible to them and to convey the impressions to the sensorium. This 

 sense, next to vision, is most valuable to man, as forming the bond of social 

 union and the great inlet for all knowledge conveyed by voice and language. 

 Each organ consists of a special nerve expanded on membranes, and en- 

 dowed with the properties of sensibility to the varying impressions of sound 

 and of conveying these impressions to the sensorium ; and secondlj^, of a 

 physical apparatus fitted for receiving, conducting, and in some measure 

 regulating these impulses in their course to the sentient nerve. 



We may divide the ear into an external, middle, and internal portion. 

 The external comprises the auricle, commonly called ear, and the external 

 meatus or tube ; the middle consists of the tympanum or drum with its 

 chain of small bones ; and the internal or true ear is called the labyrinth, 

 and comprises the vestibule, cochlea, semicircular canals, meatus auditorius 

 internus, and auditory nerve. This portion is lodged in the petrous divi- 

 sion of the temporal bone, and is thereby protected from external pressure 

 or injury. 



859 



