AUMENTARY SYSTEM OF THE FROG. 453 



firm sclerotic surrounding the eye-ball, and the sheath of 

 the optic nerve, are as usual continuous. The next layer 

 includes the vascular and pigmented choroid and the bril- 

 liant iris. Internally is the sensitive retina, while vitreous 

 humour fills the cavity behind the lens. , 



The internal ears have the usual parts and lie within the 

 auditory capsules, which are in great part bounded by the 

 pro-otics. Connecting the fenestra ovalis of the ear with 

 the tympanic membrane, which is flush with the skin, there 

 is a delicate bony rod — the columella. It lies in the 

 Eustachian tube, which opens into the mouth at the corner 

 of the gape. 



The nostrils open into small nasal cavities, with folded 

 walls of sensitive membrane ; the posterior nares open into 

 the front of the mouth. 



There are taste papillae on the tongue, and touch spots 

 on the skin. 



Alimentary System. — The frog feeds in great part on 

 insects, which it catches dexterously with its tongue. This is 

 fixed in front and loose behind. There are teeth on the 

 pre-maxillEB, maxillae, and vomers. Into the cavity of the 

 mouth the nasal sacs open anteriorly, and the Eustachian 

 tubes posteriorly. The males of Rana esculenta have a 

 pair of sacs which open into the mouth cavity at the angle 

 of the jaw, and are dilated during croaking. The tongue 

 bears numerous taste papillae. Behind the tongue on the 

 floor of the mouth is the glottis, the opening of the short 

 larynx which leads to the lungs. The larynx is supported 

 by two arytenoid cartilages and also by a ring ; with the 

 arytenoids the vocal cords are closely associated. The lungs 

 are so near the mouth that laryngeal, tracheal, and bron- 

 chial regions are hardly distinguishable. On the floor of 

 the mouth lies the hyoid cartilage, which serves for the in- 

 sertion of numerous sets of muscles. 



In the tadpole the walls of the pharynx bear gill-clefts, of 

 which no distinct traces remain in the adult. The lungs are 

 outgrowths of the gullet or oesophagus. The latter leads into 

 a tubular stomach. This is separated by a pyloric constric- 

 tion from the duodenum, or first part of the small intestine. 

 Then follow the coils of the small intestine, the wider large 

 intestine and the cloaca. 



