790 HEARING, SMELL, AND TASTE. 



orbital branches of the fifth nerve) or arise from the cervical sympathetic, 

 the afferent nerves varying according to the exciting cause. 



The act of blinking undoubtedly favors the passage of tears through the 

 lachrymal canals into the nasal sac, and hence when the orbicularis is para- 

 lyzed tears do not pass so readily as usual into the nose ; but the exact 

 mechanism by which this is effected has been much disputed. According 

 to some authors, the contraction of the orbicularis presses the fluid onward 

 out of the canals, which upon the relaxation of the orbicularis dilate and 

 receive a fresh quantity. Others maintain that a special arrangement of 

 muscular fibres keeps the canals open even during the closing of the lids, so 

 that the pressure of the contraction of the orbicularis is able to have full 

 effect in driving the tears through the canals. 



CHAPTER IV. 

 HEARING, SMELL, AND TASTE. 



695. As in the eye, so in the ear, we have to deal first with a nerve 

 of special sense, the stimulation of which gives rise to a special sensation * 

 secondly, with terminal organs through which the physical changes proper 

 to the special sense are enabled to act on the nerve ; and thirdly, with sub- 

 sidiary apparatus, by which the usefulness of the sense is increased. The 

 central connections of the auditory nerve are such that whenever the audi- 

 tory fibres are stimulated, whether by means of the terminal organs in the 

 usual way or by the direct application of stimuli, electrical, mechanical, etc., 

 the result is always a sensation of sound. Just as stimulation of the optic 

 fibres produces no other sensation than that of light, so stimulation of the 

 auditory fibres produces no other sensation than that of sound. 1 The ter- 

 minal organs of the auditory nerve are of two kinds : the complicated organ 

 of Corti in the cochlea, and the epithelial arrangements of the maculae and 

 cristse acusticse in other parts of the labyrinth. Waves of sound falling on 

 the auditory nerve itself produce no effect whatever ; it is only when by the 

 medium of the endolymph they are brought to bear on the delicate and 

 peculiar epithelium cells which constitute the peripheral terminations of the 

 nerve, that sensations of sound arise. Such delicate structures are for the 

 sake of protection naturally withdrawn from the surface of the body where 

 they would be subject to injury. Hence, the necessity of an acoustic appa- 

 ratus, forming the middle and external ear, by which the waves of sound 

 are most advantageously conveyed to the terminal organs. 



HEARING. 



696. [The ear, or organ of hearing, is composed of three parts, called 

 the external, middle, and internal ear. 



The external ear consists of an outer projecting portion, called the pinna, 

 and the auditory canal, or meatus auditorius externus. (Fig. 183.) The 

 pinna is a somewhat oblong funnel-shaped organ, the smaller portion of the 

 funnel being attached to the skull by ligamentous tissue, the larger portion 

 serving to collect and convey the sonorous undulations to the meatus. It is 

 composed of cartilage covered by integument. Its surface is irregularly 



1 It will be seen later on that there are reasons for thinking that impulses passing along 

 the auditory nerve may give rise to other effects than auditory sensations. 



