ORGAN OF HEARING. 27 



and at the same time favors the escape of the secretion from 

 the Harderian gland. 



Organ of Hearing. 



The most important part of the organ of Hearing, the labyrinth, 

 exhibits throughout the Mammalia in general a complete agreement 

 with the human structure. It is completely imbedded in the dense 

 osseous substance of the temporal bone, and in the foetus only is 

 surrounded by loose bony tissue. The direction of the semicircular 

 canals and the vestibule are in their number and situation, their 

 histological elements, and otolithic concretions, with slight variations, 

 similar to those of the human subject. In some instances, as in 

 the Ornithorynchus and the Mole, where the semicircular canals 

 are very large, they project internally into the cranial cavity. 

 Recent accurate investigations concerning the labyrinth show also 

 that in the individual genera and orders of Mammalia a number of 

 minor but very interesting differences occur. The least variations 

 occur in the vestibule of the labyrinth (which is, however, all but 

 wanting in the Whales) ; the greatest in the semicircular canals. 

 In the Cats, the Cheiroptera, and Viverridae, these canals form the 

 segment of a circle, in the Horse they exhibit a parabolic curve, in 

 the Camels, Goats, and in Myrmecophaga jubata, they form a por- 

 tion of an ellipse, frequently also of a spiral, as in the Antilopes 

 and some Edentata. In the Whales they are very small, smaller 

 than in the Field-mouse, and form a segment of a circle of scarcely 

 90. The Dromedary has the largest canals, and next to them, 

 some Seals. In many, though not in all of the Mammalia, the 

 canals open by five orifices into the vestibule. The ampullae 

 also in size and situation present numerous differences ; there are 

 nearly always, however, three ampullae present, but only two in the 

 Sloths, none being met with upon the external canal. Of all the parts 

 of the labyrinth, the cochlea varies most, namely, in the number of 

 its coils. In the Whales and Dolphins, it has only 1-J- turns, though 

 it is very large, being thus in remarkable contrast with the small 

 canals ; in Delphinus delphis it is larger than in the largest terres- 

 trial mammal, and the spires lie upon one level. In the Hedgehog 

 also the small cochlea makes only 1-J- turns, but is more conical ; 

 in the Seals, 2 coils are met with, as likewise in the Chamois. Most 

 Ruminantia, the Horse, and many of the Edentata, have not quite 

 2-J coils, which is the case too in Man, in the Apes, and Cheiroptera. 



