286 



ZOOLOGY 



SECT. 



capsule is a considerable space, the tympanic cavity (Fig. 936, tymp. 

 cav.), which communicates with the pharynx by the short Eustachian 

 tube (eus. .) already noticed (Fig. 929, eus. t.\ so that a probe thrust 

 through the tympanic membrane from outside passes directly 

 into the pharynx. In the roof of the tympanic cavity lies the 

 columella (col.), its head, or extra-columella, attached to the inner 

 surface of the tympanic membrane, its handle united to the 

 stapes (stp.), which is fixed in the membrane of the fenestra 

 ovalis (fen. ov.). Sonorous vibrations striking the tympanic mem- 

 brane are communicated by the columella and stapes to the 

 fenestra ovalis, thence to the perilymph, and thence to the 

 membranous labyrinth. The connection of the Eustachian tube 



7rte.Tftb.lcil) 



FIG. 936. Transverse section of head of Frog to show the relations of the accessory auditory 

 apparatus (diagrammatic). Skeletal structures black, with 'the exception of the columella. 

 an. tymp. annulus tympanicus ; l>. hy. body of hyoid ; buc. cav. cavity of pharynx; ch. plx. 

 choroid plexus; col. columella; eus.t. Eustachian tube; feii. ov. fenestra ovalis; med. obi. 

 medulla oblongata ; tnemb. lab. membranous labyrinth ; mnd. mandible ; Nv. VIII. auditory 

 nerve ; o. st. ornosternum ; ptg. pterygoid ; qu. ju. quadrato-jugal ; stp. stapes ; tymp. cav. 

 tympanic cavity ; tymp. m. tympanic membrane. 



with the pharynx obviates undue compression of the air in the 

 tympanic cavity. There seems little doubt that the tympano- 

 Eustachian passage is homologous with the first or hyomandibular 

 gill-cleft, although, in the Frog, it is formed independently of the 

 clefts and never opens on the exterior. 



Urinogenital Organs. The kidneys (Figs. 937 and 938, N.) 

 are flat, somewhat oval bodies, of a dark red colour, lying in the 

 posterior region of the ccelome. On the ventral face of each 

 is an elongated, yellow adrenal, and irregularly scattered 

 mphrostomcs occur in considerable numbers on the same surface ; 

 .these do not, however, communicate with the urinary tubules, 

 but with the renal veins, and serve to propel the lymph from the 

 coelome to the venous system. The ureters (Ur.) pass backwards 



