A TURTLE 143 



panic cavity. With a sharp scalpel cut out the tympanic membrane 

 around its edge and, lifting it up, note the delicate, rodlike colu- 

 mella, which will be seen extending inward from it. The columella 

 is a bone which has cartilaginous ends. Its function is to conduct 

 the sound waves to the inner ear. Snip away the edges of the tym- 

 panic membrane and finally cut it from the columella, exposing 

 the cavity of the middle ear. 



This will be seen to be a large space which extends back from 

 the opening to the hinder end of the skull. Probe it. The columella 

 extends from the tympanic membrane to a point on the ventral 

 wall of the cavity, where it will be seen entering a long canal just 

 large enough to contain it; at the inner end of this canal the 

 columella expands and is applied to the fenestra ovalis, the open- 

 ing into the inner ear. The inner end of the columella and the 

 fenestra ovalis will not be seen. Just beneath the columella is the 

 slitlike inner opening of the Eustachian tube; probe it with a 

 bristle and find its opening into the pharynx. 



The inner ear, or membranous labyrinth, will not be studied in 

 this dissection. 



Exercise 17. Draw a diagrammatic sketch of the tympanic cavity on 

 a large scale. 



The central nervous system and the cranial nerves will be 

 studied in another animal. 



The Skeletal System. This system of organs is made up of two 

 portions,— the exoskeleton, which is dermal in origin, and the 

 endoskeleton, which is the internal bony and cartilaginous frame- 

 work of the body. 



The exoskeleton consists of the horny dermal plates (the tor- 

 toise shell) and the scales which cover the external surface of the 

 body, and also of the bony dermal plates which underlie the horny 

 plates and help form the shell. The bony portion of the shell is 

 not composed of these dermal bones alone but includes also cer- 

 tain portions of the endoskeleton ; namely, the trunk vertebrae, 

 most of the ribs, and perhaps the clavicle, which are more or less 

 completely fused with the dermal bones. The exoskeleton includes 

 also the claws and the horny covering of the jaws. 



