278 



ZOOLOGY 



SECT. 



CvAo 



FK 



panic membrane and internally by the outer wall of the auditory 

 capsule is a considerable space, the tympanic cavity (Fig. 947, tymp. 

 cav.), which communicates with the pharynx by the short Eusta- 

 chian tube (eus. t.) already noticed (Fig. 940, 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 membrane are com- 

 municated by the columella and 

 stapes to the fenestra ovalis, thence 

 to the perilymph, and thence to the 

 membranous labyrinth. The con- 

 nection of the Eustachian tube with 

 the pharynx obviates undue com- 

 pression of the air in the tympanic 

 cavity. There seems little doubt 

 that the tympano-Eustachian pas- 

 sage is homologous with the first 

 or hyomandibular gill-cleft, although, 

 in the Frog, it is formed indepen- 

 dently of the clefts and never opens 

 on the exterior. 



Urinogenital Organs. - The 

 kidneys (Figs. 948 and 949, 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 

 nephrostomes 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 from the outer 

 borders of the kidneys and open into the dorsal wall of the 

 cloaca (Cl.). The kidney is developed from the mesonephros of 

 the embryo, the ureter from the mesonephric duct. In the larva 

 a large pronephros is present and is, for a time, the functional 

 kidney. 



Opening into the cloaca on its ventral side is an organ 

 (Fig. 940, bl.) mentioned in the general account of the Craniata 

 (p. 116), but here actually met with for the first time. It is a 

 bilobed, thin-walled, and very delicate sac into which the urine 



Fio. 948. Rana esculenta. Urino- 

 genital organs of the male. Ao. dorsal 

 aorta ; CL cloaca ; Co. postcaval vein ; 

 FK, fat-bodies ; HO, testes ; N, kidneys ; 

 S, 8', apertures of ureters into cloaca ; 

 Ur. ureters. (From Wiedersheim's Com- 

 parative Anatomy.) 



