4i8 



CHORDATE ANATOMY 



In reptiles the lagena and basilar papilla unite in an elongated cochlea, 

 which in crocodiles becomes spirally wound. By the attachment of the 

 cochlear duct to the bony labyrinth along two sides, the perilymphatic 

 cavity is divided into two portions, the scala vestibuli and the scala 

 tympani. Further advance towards the mammalian ear is seen in the 

 appearance of a membrane-covered window, the fenestra vestibuli, to 

 which the stapes is attached. As a result of these advances, the hearing 

 of reptiles is noticeably keener than that of creatures lower in the scale. 



In mammals, the length of the cochlea varies from a half-turn in 

 Echidna to three and a half turns in the deer. The keenness and range 



Fig. 367. — The left membranous labyrinth of vertebrates as seen in lateral view. A , 

 Myxine; B, Petromyzon; C, teleost; D, frog; E, crocodile; F, bird; G, mammal. The 

 series represents fairly closely an evolutionary series. (Redrawn after Hesse.) 



of the sense of hearing differ correspondingly in the two animals. In 

 correlation with the elongation of the cochlea, the length of Corti 's organ 

 is increased and, with it, the range of audible sounds. A second mem- 

 brane-covered window, the fenestra cochleae, is added to the inner ear. 

 The malleus and incus are added to the stapes, to form a chain of bones so 

 arranged that the amplitude of the vibrations of the ear drum is reduced 

 and their intensity increased as they pass from the drum to the fenestra 

 vestibuli. The efficiency of the apparatus is increased by the addition 

 of two muscles, the stapedial, the smallest of skeletal muscles, and the 

 tensor tympani which tightens the drum. (Fig. 164) 



The conclusion of morphologists that the vertebrate membranous ear 

 is a modiiied lateral-line organ, or a group of such organs, seems justified 

 by the fact that the membranous ear develops, like a lateral-line organ, 



