SENSIBILITY OF THE IXTKKXAI, OEGANS 111 



The above discussion is necessary to give the student s 

 idea of the dill'crcnt sensory fax -tors of central nr peripheral origin, 

 which necessarily enter into the formation of the so-called active 

 tactile perceptions (in which the cut a neons sensations are associate! I 

 with a preponderance of various kinaesthetic elements) and of the 

 different values ascribed by the physiologists who have studied 

 this dithcnlt subject to the various factors concerned in the dis- 

 crimination of weights. 



IX. In Chapter VII. of the last volume we discussed the 

 Hind-brain at length as the seat of the organs of subconscious 

 sensations, on which the normal tone of the muscles largely 

 depends. We saw that these subconscious sensations are main- 

 tained by a number of afferent paths which are in direct or 

 indirect relation with the cerebellum and spinal bulb. Of these. 

 afferent paths we emphasised the importance of those represented 

 by the vestibular roots of the eighth cerebral nerves, by which 



C. 



FIG. 44. Model of the left labyrinth of human ear; A, from outer side; /:, from inner side; 

 C, from aticive. 'j. (Henle.) s., superior; p., posterior; <., external (lateral), semicircular 

 canal ; a., ampullae ; ,:./., aqueduct of \estibule ; f.o., fenestra ovalis (vestilmli); /.;., fenestia 

 rotunda (cochleae); /., coiled tube of cochlea. 



the so-called non-acoustic labyrinth is innervated (Vol. III. p. 461). 

 At this point we may discuss the many experimental facts that 

 have been collected with reference to this most delicate peripheral 

 sense-organ, and the various theories put forward for their 

 interpretation. A full account would, however, exceed the limits 

 of this text-book, and we must confine ourselves to discussing 

 the most important to our own point of view. 



We must begin with a brief description of the anatomy of 

 the Internal Ear or Labyrinth, referring the reader for greater 

 detail to anatomical text-books. 



The internal ear is morphologically divided into two parts 

 the Cochlea, innervated by the ramus cochlearis, and the Vestibu- 

 lum, consisting of the three semicircular canals, the utricle and 

 the saccnK innervated by the vestibular branch of the eighth 

 nerve. Physiologically, too, this division seems to be justified. 

 (See Vol. III. p. 405.) 



The cochlea is a later formation than the vestibular organs. 

 In fishes it is quite rudimentary, and is represented merely by 

 the lagtna, which is a small appendage of the saccule. In 



