290 ORGANS OP THE SENSES. 



peri-lymph from its osseous parieties. The auditory nerves, 

 as in other vertebrated animals, spread on the ampullae of 

 the semi-circular tubes and supporting the cretaceous lapilli 

 of the median sinus and the succulus, can receive their im- 

 pressions only from the undulations of this fluid medium. 

 The minute component crystals of the two cretaceous lapilli 

 of the vestibule are loosely aggregated together as in the 

 cartilaginous fishes and in all the higher vertebrata, and 

 the membranous labyrinth is separated from the fenestra 

 ovalis by the peri-lymph and by the membrane of that 

 orifice, so that this thin enveloping peri-lymph receives 

 the sonorous vibrations from the tympanum and transmits 

 them to the sensitive parts of the membranous labyrinth. 

 The aqueducts appear to be but vascular foramina. The 

 anterior auditory nerve, accompanied by the facial, passes 

 to the two anterior ampullae and to the cretaceous lapillus 

 of the median sinus, and the posterior auditory branch 

 passes to the posterior ampulla, to the saccular lapillus, 

 and to the cochlea, as in the inferior vertebrated classes. 

 So that there is great uniformity of plan in the structure 

 of this delicate acoustic organ from the simple vestibule 

 of the articulata to its most complex form in the highest 

 mammalia, where the different densities and forms of the 

 pulpy granular lapilli, the gelatinous ento-lymph, and the 

 thin fluid of Cotunnius, regulate and limit each others vi- 

 brations, and approximate the phenomena of hearing to 

 the undulations of light through the various humours of 

 the visual organs. 



FOURTH SECTION. 



Organs of Smell. 



The organs of smell, which are destined to receive and 

 transmit the impressions of odorous particles diffused through 

 the medium in which animals live, are much simpler in 

 their structure, and more difficult to determine, in the in- 

 ferior tribes of animals than those of sight or hearing, and 

 they appear also to be less important for the preservation of 

 life, and less general in their occurrence throughout the 

 animal kingdom. It does not appear that the radiated 



