THE SENSES. 191 



lions, where the ossicula are far smaller relatively, and less solid than in 

 whales, manatees, and the earless true seals, there are well-formed, mov- 

 able external ears. The ossicula seem to be vestigial relics utilized for 

 the auditory function. In land animals they vary in shape according to 

 the type of the animal rather than in relation to its acuteness of hearing. 

 I have never found a muscular laxator tympani in any animal, but the 

 tensor exists as a ligament in whales where the malleus is fixed. " (Alban 

 Doran. ) 



Action of the Stapedius. The influence of the stapedius muscle in 

 hearing is unknown. It acts upon the stapes in such a manner as to 

 make it rest obliquely in the fenestra ovalis, depressing that side of it on 

 which it acts, and elevating the other side to the same extent. It pre- 

 vents too great a movement of the bone. 



Functions of the Fluid of the Labyrinth. The fluid of the laby- 

 rinth is the most general and constant of the acoustic provisions of the 

 labyrinth. In all forms of organs of hearing, the sonorous vibrations affect 

 the auditory nerve through the medium of liquid the most convenient 

 medium, on many accounts, for such a purpose. 



The crystalline pulverulent masses (otoliths) in the labryinth would 

 reinforce the sonorous vibrations by their resonance, even if they did not 

 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 impulses 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 prolongations 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 consider- 

 able extent for their due performance upon a correct notion of our equili- 

 brium, that the semicircular canals are connected in some way with this 



