478 Comparative Morphology of Chordates 



fits into an oval aperture (fenestra ovalis) in the bony wall of the 

 auditory capsule, the part of the cranium containing the internal ear. 

 (Fig. 353). The stapes is attached by membrane to the edge of the 

 "oval window" and is, therefore, capable of a certain small amount of 

 movement in and out as the columella transmits external vibrations 

 to it. The columella, cartilaginous in the embryo, may ossify to form 

 two, or even three, connected parts. A second "window," the fenestra 

 rotunda, pierces the bony wall of that part of the auditory capsule 

 against which the tympanic cavity abuts, but it is a closed window — 

 closed by a taut membrane. This "round window" serves to facilitate 

 the passing of vibrations through the watery fluid (perilymph) which 

 fills the considerable space intervening between the delicate auditory 

 sac (internal ear) and the wall of the bony capsule within which it lies. 

 The otic sac is not only immersed in perilymph but is filled by a similar 

 endolymph. As the vibrating columella pushes the stapes slightly 

 inward against the perilymph, the membrane filling the fenestra 

 rotunda may bulge slightly outward, and meanwhile the vibrations 

 will have affected all the fluids, endolymph as well as perilymph, within 

 the auditory capsule. 



In the New Zealand tuatara (Sphenodon) there are well-developed 

 tympanic cavities and the usual columellar apparatus, but no definitely 

 differentiated tympanic membrane. The tympanic cavity is covered 

 externally by ordinary thick skin containing a very small cavity which 

 may be interpreted as a vestige of an external auditory cavity. 



In snakes tympanic cavity and membrane are lacking. There 

 is, however, a somewhat reduced columella which is merely embedded 

 among the muscles adjacent to the auditory capsule and in some cases 

 is connected by ligament to the quadrate, the skull bone to which the 

 lower jaw is articulated. In the externally snakelike legless lizards, the 

 tympanic mechanism is in a more or less reduced condition. The feed- 

 ing habits of snakes probably account, at least in part, for the absence 

 of the tympanic mechanism. A snake may take a month's ration of 

 food at one mouthful, as when a rattlesnake swallows a rabbit. An 

 extremely loose-jointed jaw-skeleton makes this possible. In the ab- 

 sence of a sternum, the ribs are free to swing outward. As the prey 

 passes from mouth to stomach, the body-wall of the intervening region 

 is severely stretched. The normal position for the tympanic apparatus 

 is immediately behind the jaws. It is clearly not a safe place for the 

 delicately articulated columellar mechanism. 



The reptilian brain is larger, relative to the body, than the brains 

 of fishes and amphibians. All five regions of the brain are strongly 

 developed and exhibit more internal complexity than is found in brains 



