xxv] 



INTERNAL EAR 



649 



numerous short hairs. Underlying the sensory epithelium between 

 the divergent legs of the rods of Corti and beyond them is stretched 

 a connective tissue membrane consisting of parallel fibres termed 

 the basil a r membrane, and one theory of hearing is that these 

 fibres vibrate like piano strings in harmony with tones of different 

 pitch and stimulate the hair cells above them. The roof of the 

 cochlea is a thin layer of cells termed Reissner's membrane (10, Fig. 

 320), and from the inner side of the cochlea a flap termed the 

 membrana tectoria projects into its cavity. This is supposed 

 to act as damper to the vibrations of the sense-hairs when these 

 vibrations become excessive. The 

 cochlea, often termed the mem- 

 branous cochlea or scala media, 

 is enclosed in an expansion of the 

 peri-otic capsule of the same shape 

 termed the osseous cochlea. In- 

 side the osseous cochlea run two 

 lymph spaces, one lying above the 

 membranous cochlea, termed the 

 scala vestibuli, and the other 

 below the membranous cochlea, 

 termed the scala tympani. These 

 two spaces communicate with one 

 another round the tip of the mem- 

 branous cochlea. The scala tympani 

 ends externally against a membra- 

 nous window in the peri-otic bone 

 termed the fenestraovalis. There 

 is another membranous window in 

 the same bone called the fenestra 

 rotunda, behind which lies the scala 

 vestibuli and against which the stapes 

 impinges externally, and when a 

 vibration strikes the ear-drum, it is 

 transmitted by malleus, incus and 



stapes to the fenestra rotunda and hence when it reaches the top of 

 the cochlea to the scala tympani. Along the lymph in this it comes 

 till it reaches the base of the cochlea. As the pulse passes along the 

 scala tympani it is transmitted through Reissner's membrane to the 

 cavity of the sense organ or scala media. Thus it stimulates the 

 hair cells which respond to tones of different pitch and hence 



FIG. 321. Sternum and sternal ribs 

 of a Dog, Canis familiar is x . 



1. Presternum. 2. First sterne- 

 bra of mesosternum. 3. Last 

 sternebra of mesosternum. 



4. Xiphisternum. The flattened 

 cartilaginous plate terminating 

 the xiphisternum is not shown. 



5. First sternal rib. 



