SENSE ORGANS OF FROG 66l 



(9) Glossopharyngeal, to the tongue and some of its muscles ; with 

 a ganglion which unites with that of the tenth ; 

 (10) Vagus, with branches to lungs, heart, stomach, etc. 



The spinal cord gives origin to ten pairs of spinal nerves, and is 

 swollen at the origin of those which go to the limbs. Around the 

 union of the anterior and posterior roots lie sacs with crystals of 

 carbonate of lime. 



The sympathetic system consists of about ten pairs of ganglia — (a) 

 united by branches to the spinal nerves ; (6) united to one another by 

 longitudinal trunks which accompany the dorsal aorta and the systemic 

 arches, and end anteriorly in the Gasserian ganglion ; (c) giving off 

 branches to the heart, the aorta, and the viscera in the pelvic region. 



Sense organs. — The eyes project on the top of the head 

 and on the roof of the mouth. There is a third eyehd. 

 The transparent cornea in front, the firm sclerotic surround- 

 ing the eyeball, and the sheath of the optic nerve, are as 

 usual continuous. The next layer includes the vascular 

 and pigmented choroid and the brilliant 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. This 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 

 premaxillae, maxillae, and vomers. Into the cavity of the 

 mouth the nasal sacs open anteriorly, and the Eustachian 

 tubes posteriorly. The males oi Rana esculenta have a pair 

 of resonating 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 sup- 



