AUTONOMIC SYSTEM 389 



the subclavian artery. In front the longitudinal cord enters the 

 skull with the ninth and tenth nerves, is connected with the tenth, 

 and ends in the Gasserian ganglion. From the sympathetic ganglia 

 small nerves are given off to those of the opposite cord and to 

 the blood vessels and viscera. These nerves break up and rejoin 

 to form networks or plexuses. 



SENSE ORGANS 



The eyes (Fig. 304) have the same general structure as those 

 of mammals, and the image is formed in the same way. The 



nm 



Fig. 304. — The amphibian eye and its accommodation. — From Young, The Life 

 of Vertebrates, 1950. Clarendon Press, Oxford. After Franz and Beer. 



A, Anuran eye in vertical section ; B, anterior part of eye of toad {Bufo) in relaxation ; C, the same in 



accommodation. 

 ac, Macular region ; io, obliquus inferior ; ir, rectus inferior ; //, lower lid ; /m, protractor lens muscles; 



n, optic nerve ; nm, nictitating membrane ; pn, border of pupil ; sc, sclerotic ; so, obliquus superior; 



sr, rectus superior ; ul, upper lid ; z, suspensory ligaments. 



movable lower hd is, like that of the dogfish, a nictitating 

 membrane, or third eyehd. Accommodation for near vision is 

 achieved by protractor lentis muscles, which pull the lens forward. 

 Both rods and cones are present in the retina, and there is a 

 macula where the cones are much more numerous than the rods. 



