222 



ANATOMY OF VERTEBRATES. 



the condition of pulverulent aggregates of combined carbonate 

 and phosphate of lime, the latter in greater proportion in Mammals 

 than in Fishes: the particles are held together by a mucous 

 tissue. The membranous labyrinth has a ciliate inner surface, 

 and contains a thinner t endolymph ' than in fishes : it is suspended 

 in the common serosity of the bony labyrinth, which is distin- 

 guished as ( perilymph.' 



Taking a retrospect of the course of the ear-organ, the primitive, 

 constant, and essential element is the ' sacculus,' fig. 165, e,f, with 

 its otolites, which receive the proportion of the nerve-supply not 

 resolved into the pulpy lining of their bag : this simple condition 

 obtains in Cephalopods. 1 In the Myxine something like one bent 

 canal, and in the Lamprey two, are continued from the sacculus : 

 in all higher Vertebrates the three semicircular canals are 

 established ; but in most fishes they float, as shown in vol. i. 

 p. 342, fig. 227, in the common serosity of the cranium ; their 

 special bony cases, intercepting so much of the arachnoid fluid as 

 now forms the ( perilymph,' are subsequently developed : finally 

 is added the complex cochlea, into which the primitive mem- 

 branous labyrinth is not extended. 



In fishes, where the acoustic nerves are affected by vibrations 

 of the endolymph propagated from the cranium or the body gene- 

 rally, the otolites are large, and usually of crystalline density. In 

 air-breathers the sonorous vibrations of the atmosphere are more 

 directly received : they first strike upon a membrane set in a frame 

 and stretched across the opening of an air-chamber, like a drum. 

 In Mammals the e membrana tympani ' is more delicate than in 

 Reptiles, and, with few exceptions, is concave outwardly. Sound 

 is usually collected into a conch, the hollow of which can be 

 directed to its source. The medium of acoustic communi- 

 cation between the drum-membrane and the labyrinth under- 

 goes also, in Mammals, that instructive 

 course of ossification and development 

 which converts the avian columella and 

 its cartilages into the chain of little 

 bones called e otosteals.' These, after 

 the external ear, are the seat of the 

 chief modifications of the organ in the 

 present class. They retain, in Mam- 

 malia, the names suggested by their 

 shape in Man, viz. { stapes,' fig. 166, C, 

 ( incus,' b, and i malleus,' A : a small 

 epiphysis between b e and c a, when separate, is the ' orbiculare \ 



1 civ. p. 294, and note. OCX L IX, p. 619. 



1G6 



Human otosteals, magn. xcvir 



