12SS ORGANS OP THE SENSES. 



externally with a copious transparent, thin and colourless 

 peri-lymph which separates it from the periosteum, lining the 

 interior parietes of the osseous labyrinth. The membranous 

 semi-circular canals and their ampullae are comparatively 

 slender, their median portion forms the larger part of the 

 broad, short, and irregular vestibule, and its lapillus corres- 

 ponds with the utricular of fishes. The sacculus vestibuli 

 and its lapillus are proportionally small, and the cochlea 

 forms a large turbinated cavity divided longitudinally 

 throughout by an internal solid spiral lamina, forming from 

 two to four spiral turns ; but in the ornithorhyncus it is as 

 simple as in birds. As in the inferior vertebrata the exterior 

 semi-circular canal extends outwards horizontally and at 

 right angles to the other two, and the anterior and posterior 

 unite together to form a common canal before entering the 

 vestibule, their direction being nearly vertical with relation 

 to the floor of the cranium. The nerves of the membranous 

 labyrinth are chiefly confined to the ampullae of the semi- 

 circular canals and to the two sacs containing lapilli, and 

 the two lapilli are proportionally larger in the foetus than 

 in the adult. The osseous spiral lamina dividing longi- 

 tudinally the interior of the convoluted cochlea is still 

 membranous at its peripheral margin, and the scala vestibuli 

 communicates freely with the scala tympani at its dilated apex 

 where it receives a branch of the acoustic nerve, but no 

 nervous brandies are perceived on the membranous semi-cir- 

 cular canals. The two vestibular openings vary much in their 

 form and size in different mammalia, and the aqueducts of the 

 vestibule and cochlea appear connected, the one inter- 

 nally with the dura mater and the other externally with 

 the periosteum. The cochlea is of a narrow lengthened 

 spiral form in many of the rodentia, more short and orbi- 

 cular in the cetacea, and in most of the higher orders of 

 quadrupeds it forms a turbinated cavity with about two turns 

 and a half as in the human ear. The tympanic cavity, im- 

 bedded in the temporal bone, communicating by several aper- 

 tures with the mastoid cells, and by a lengthened ossified 

 Eustachian tube with the fauces, is bounded by a thin fibrous 

 membrana tympani concave externally, and contains four 

 ossicula articulated moveably with each other and provided 

 with distinct muscles for their movements. The external 



