300 



COMPARATIVE ANATOMY 



In Chelonians an external and an internal portion of the 

 tympanic cavity can be distinguished : the former is surrounded 

 by the quadrate, which is correspondingly hollowed out and covered 

 externally by the tympanic membrane. The two portions com- 

 municate by a canal through which the columella extends, and 

 which is bounded either entirely by the quadrate or partially by 

 soft parts also. 



In many Reptiles the distal end of the ductus endolymphaticus 

 is situated close under the roof of the skull beneath the parieto- 

 occipital suture, and in the Ascalabota the duct even leaves the 

 cranial capsule, passes back between the muscles of the neck, and 



ca 



mn 



ap 



FIG. 219. RIGHT MEMBRANOUS LABYRINTH OF A, Lncer/a riritlix, AND 15, 

 Alligator mississippiensis, from the outer side. (After G. Retzius.) 



ade, aperture of the ductus endolymphaticus ; rsc, sacculo-cochlear canal ; frl, 

 foramen recessus scala? tympani ; mb, membrana basilaris : w, septum 

 cruciatum ; le, tegmentum vasctilosum. (Other letters as in Figs. 210 and 

 217.) 



in the region of the pectoral arch becomes swollen to form a large 

 folded sac, from which finger-shaped processes extend to the 

 ventral surface of the vertebral column and to the sub-mucous 

 tissue of the pharynx. These processes may also branch out in a 

 labyrinthic manner into the orbit : they are filled with a white 

 semi-solid mass of otolithic substance, as is the endolymphatic 

 duct of all Vertebrates, at any rate in the embryo. In Birds, the 

 duct does not pass out of the cranial cavity. 



A tympanic membrane is present in Birds and all Reptiles 

 except Hatteria, Snakes, and Amphisbaenians, in which the tym- 

 panum and Eustachian tube have undergone degeneration. 



