VERTEBRATES. 305 



through sclerotic, choroid, and retina, and is then distrib- 

 uted over the inner surface of the latter layer. 



The eyeball is moved by six muscles, which are essentially 

 alike in all vertebrates. Four of these are straight or 

 rectus muscles, two are oblique. These muscles are con- 

 trolled by the three eye-muscle nerves (p. 300). 



The alimentary canal runs through the body from mouth 

 to vent. In it several parts can be distinguished. 



The mouth, at or near the anterior end, is without 

 fleshy lips, except in the mammals. The mouth is fre- 

 quently armed with teeth, and even in those groups, like 

 the turtles and the birds, where teeth are absent the 

 germs occur in the young, a fact which points to the 

 descent of these from toothed ancestors. 



The tongue is formed as a fold of the floor of the mouth, 

 and is usually supported by a skeleton (hyoid bone, p. 

 294) derived from the first or first and second visceral 

 arches. In some it is without powers of motion, but 

 frequently it is very mobile. Usually it is attached 

 behind, the front margin being free, but in many Am- 

 phibia it is attached in front and folded back in the mouth. 



The mouth-cavity is succeeded by the pharynx, a re- 

 gion concerned in respiration and distinguished by contain- 

 ing the respiratory openings (internal nostrils, gill-slits, 

 glottis). 



Behind the pharyngeal region is the digestive tract 

 proper. In some vertebrates it is scarcely possible to dis- 

 tinguish regions in it, but in most cases several distinct 

 portions occur. Those usually to be recognized are the 

 following : 



The pharynx communicates with the gullet or oesopha- 

 gus, a muscular tube which frequently serves only to 

 carry food back to the stomach, On the other hand, a 



