60 THE MUSCULAR SYSTEM OF THE FROG 



iv. The masseter is a small muscle placed behind the 

 temporalis : it arises from the quadratojugal and 

 runs downwards and slightly backwards to be 

 inserted into the outer surface of the mandible, 

 just in front of the joint. 



To see the insertions of these last three muscles the mouth should 

 be opened widely. 



3. Muscles of the eyeball. 



Remove the temporal and pterygoid muscles ca-refuUy, dissecting 

 them away from their origins, and then turning the muscles down 

 and cutting them short close to their insertions. Remove also the 

 lower jaw ; pin the frog out on its back and dissect away carefully 

 the mucous membrane of the roof of the mouth. 



i. The levator bulbi is a thin sheet of muscle lying 

 between the mucous membrane and the eye. Its 

 fibres arise from the side of the skull, run out- 

 wards underneath the eye, and are inserted into 

 the upper jaw. The muscle by its contraction 

 serves to lift up the eyeball and so make it more 

 prominent. Some of its fibres are inserted into 

 the lower eyelid, which they serve to depress, 

 acting as a depressor palpebrae inferioris. 



Remove the levator bulbi and clean the remaining muscles, 

 dissecting them partly from the dorsal and partly from the 

 ventral surface. 



a. The recti muscles are a group of four small muscles 

 which arise close together from the inner and posterior 

 angle of the orbit close to the optic foramen, and run 

 forwards and outwards, diverging f rom one another, to 

 be inserted into the bulb of the eye. 



i. The rectus superior is inserted into the dorsal 

 surface of the eyeball : it is seen best from above. 



ii. The rectus externus, the most posterior of the 

 four, is inserted into the posterior surface of the 

 eyeball : it is seen best from the side or from 

 below. 



iii. The rectus internus, the longest of the four, runs 

 forwards between the skull-wall and the eyeball, 



