THE SPECIAL ANATOMY OF THE FROG 57 



In studying muscles separate each one from its fellows as carefully as possible 

 with a forceps or probe, and determine the origin, insertion, and general action 

 of each. 



a) Muscles of the lower jaw: Pull off all tissue between the eye and tympanic 

 membrane. Pull off the tympanic membrane, identifying underneath it a circu- 

 lar cartilage, the tympanic ring, resting upon the squamosal bone. Between 

 the tympanic ring and the eye a mass of muscles will be found passing to the 

 lower jaw, on the inner side of the posterior end of the upper jaw. Remove the 

 end of the upper jaw (quadratojugal bone) so as to reveal the complete course 

 of these muscles. The following three may be readily identified: 



(1) The temporal muscle arises from the side of the skull and passes down 

 between the eye and the tympanic ring to be inserted on the posterior end of the 

 lower jaw. Action, closes the mouth (elevator of the jaw). 



(2) The masseters originate from the tympanic ring and adjacent bones and 

 are inserted on the lower jaw behind the temporal. Action, same as preceding; 

 also stretch the tympanic membrane. 



(3) The depressor mandibuli is a muscle of the jaw located behind the 

 tympanic ring. It arises from the tympanic ring and from the general fascia 

 of the back (dorsal fascia) and passes to the extreme posterior tip of the lower 

 jaw, where it is fastened to Meckel's cartilage. Action, opens the mouth 

 (depressor of the jaw) and stretches the tympanic membrane. Pull the lower jaw 

 fully open to see more clearly the arrangement of these three muscles. 



b) Muscles of the dorsal side of the trunk: This portion of the body is more 

 or less covered by the dorsal fascia, a strong membrane fastened to the ilium 

 bones, the neural arches of the vertebrae, and the skull, and furnishing a place 

 of insertion of many muscles. Easily identifiable muscles of the back beginning 

 behind the ear are: 



(1) The dorsalis scapulae is the anterior half of the large triangular mass 

 behind and partially covered by the depressor mandibuli. Origin, from the 

 dorsal margin of the suprascapula; remaining course like the next. 



(2) The latissimus dorsi is the posterior portion of the triangular mass. 

 Origin, dorsal fascia; unites with the preceding to be inserted on the deltoid 

 ridge of the humerus; action, raises the fore limb upward and backward (abduc- 

 tion of the limb). 



(3) The longissimus dorsi is the long muscle extending from the anterior 

 third of the urostyle forward to the skull. Remove the dorsal fascia and pre- 

 ceding muscles to see its full course. It is attached at many places to the 

 vertebrae. Action, raises the head and straightens the back. 



(4) The coccygeo-sacralis runs diagonally from the urostyle just behind the 

 insertion of the preceding muscle to the transverse process of the sacral vertebra. 

 Pull off the remainder of the dorsal fascia to see it. Action, draws the back 



