580 HANDBOOK OF PHYSIOLOGY. 



actually touch the membranes upon which the nerves are expanded; but, 

 inasmuch as these bodies lie in contact with the membranous parts 

 of the labyrinth, and the vestibular nerve-fibres are imbedded in them, 

 they communicate to these membranes and the nerves, vibratory im- 

 pulses of greater intensity than the fluid of the labyrinth can impart. 

 This appears to be their office. Sonorous undulations in water are not 

 perceived by the hand itself immersed in the water, but are felt distinctly 

 through the medium of a rod held in the hand. The fine hair-like pro- 

 longations from the epithelial cells of the ampullae have, probably, the 

 same function. 



Functions of the Semicircular canals. Besides the function of 

 collecting in their fluid contents sonorous undulations from the bones of 

 the cranium, the semicircular canals appear to have another function less 

 directly connected with the sense of hearing. Experiments show that 

 when the horizontal canal is divided in a pigeon a constant movement of 

 the head from side to side occurs, and similarly, when one of the vertical 

 canals is operated upon, up and down movements of the head are ob- 

 served. These movements are associated, also, with loss of co-ordination 

 as after the operation the bird is unable to fly in an orderly manner, but 

 flutters and falls when thrown into the air, and, moreover, is able to feed 

 with difficulty. Hearing remains unimpaired. It has been suggested, 

 therefore, that as loss of co-ordination results from section of these 

 canals, and as co-ordinate muscular movements appear to depend to a 

 considerable extent for their due performance upon a correct notion of 

 our equilibrium, that the semicircular canals are connected in some way 

 with this sense, possibly by the constant alterations of the pressure of 

 the fluid within them: the change in the pressure of the fluid in each 

 canal which takes place on any movement of the head, producing sensa- 

 tions which aid in forming an exact judgment of the alteration of posi- 

 tion which has occurred. 



Functions of the Cochlea. The cochlea seems to be constructed 

 for the spreading out of the nerve-fibres over a wide extent of surface* 

 upon a solid lamina which communicates with the solid walls of the 

 labyrinth and cranium, at the same time that it is in contact with the 

 fluid of the labyrinth, and which, besides exposing the nerve-fibres to the 

 influence of sonorous undulations, by two media, is itself insulated by 

 fluid on either side. 



The connection of the lamina spiralis with the solid walls of the 

 labyrinth, adapts the cochlea for the perception of sonorous undulations 

 propagated by the solid parts of the head and the walls of the labyrinth. 

 The membranous labyrinth of the vestibule and semicircular canals is 

 suspended free in the perilymph, and is destined more particularly for 

 the perception of sounds through the medium of that fluid, whether the 

 sonorous undulations be imparted to the fluid through the fenestrae, or 



