VII.] 



THE COMMON FROG. 



and they may be so inserted either by their own 

 muscular fibres or by the intervention of a tough 

 membrane or a dense fibrous cord called a " tendon." 



Fig. 59. — Superficial Muscles of the Perch. The fin-rays of all the fins are cut off. 

 I, great lateral muscle, showing the numerous vertical tendinous intersections 

 slightly but variously inflected ; 2, small superficial muscles inserted into the fin- 

 rays of the dorsal and ventral fins ; slender longitudinal muscle running (in the 

 interval of the summits of the two great lateral muscles) between the dorsal and 

 caudal fins ; 5, similar muscle on the ventral margin, which also appears between 

 the anal and ventral fins ; 6, small radiating muscles of the caudal fin ; 7, part of 

 the great lateral muscle inserted into the skull ; 8 and 9, elevators of the oper- 

 culum ; 10, elevator of the palato-quadrate arch ; 11 and 12, muscular mass by 

 which its contraction closes the jaws ; 13, superficial muscles of the pectoral fin ; 

 14 and 15, muscles of the ventral fin. 



All the motions of an animal are produced by 

 means of the contractions of its muscles pulling the 

 bones, which act as so many levers (of different kinds 

 according to circumstances), and so effecting loco- 

 motion. 



These muscular contractions are in life produced 

 by the agency of certain of the nerves proceeding 

 from the nervous centres, i.e. from the brain and 

 spinal marrow, and which carry an influence outwards 

 to the muscles. Other of the nerves so proceeding 

 convey an influence inwards to the nervous centres 

 from an irritated portion of the body's surface. 



The muscles, however, especially in the frog, may, 

 for a time, be made to contract after death by direct 

 irritation of the nerves themselves. 



