868 



COMPARATIVE ANATOMY 



The structures of the internal ear just mentioned form the mem- 

 branous labyrinth, which is filled with a fluid called endolymph, in which 

 a number of solid particles, called otoliths, are found. The otoliths are 

 very tiny crystals of calcium carbonate which cause the endolymph to 

 have a milky appearance. In the true fishes these tiny crystals have 

 become aggregated into one or more ear stones of considerable size. In 

 the dogfish, as the endolymph duct has an opening to the outside, sand 

 from the exterior may also form part of the otoliths. A: 



Fig. 488. 



Diagrams of ethmoturbinals in Mam- 

 mals. A, Type showing endoturbinalia 

 alone. B, Type with endoturbinalia (heavy 

 lines) and two ranks of ectoturbinalia. 

 C, Turbinals with pneumatic cavities in 

 the ox. D,E,F, three actual cases in man, 

 showing individual variation. (From Wilder 

 after Paulli.) 



Fig. 489. 



Lateral wall of nasal cavity of man. 

 eg., .crista galli ; ci.cin.cs., inferior, middle 

 and superior conchae ; fpn., foramen pala- 

 tinum majus ; fsp., sphenopalatine fora- 

 me*!! ; ic., incisive canal ; osm., opening of 

 maxillary sinus ; af, frontal sinus ; ss., 

 sphenoidal sinus. (After Corning.) 



Cartilage then appears, practically covering the membranous laby- 

 rinth. This becomes the skeletal (in higher forms the bony) labyrinth, 

 and is separated from the membranous labyrinth by a small space filled 

 with a fluid called perilymph. As ossification then takes place in the 

 cartilage* the ear bones are formed (Fig. 487). There are two openings 

 into the middle ear, the fenestra tympani, also called the fenestra rotunda, 

 which is closed by a membrane, and the upper opening, known as the 

 fenestra ovale or the vestibule. The stapes (Fig. 487), a small piece of 

 cartilage or bone, is held in place by the membrane. 



The lagena grows quite rapidly and coils up into a spiral, in fact, 

 it extends to the skeletal labyrinth, thus dividing the perilymph space 

 ihtb two spiral tubes, called scalae (Fig. 486). The upper one is. called 

 the scala vestibuli, and the lower the scala tympani, while the lagena 

 itself forms the scala media, also called the cochlear duct. The bony 

 labyrinth which contains the scala is named the cochlea. > 



As the cochlea only develops in the higher forms,; and as our own 



