THE SPECIAL ANATOMY OF THE FROG 59 



hyoid. It is readily seen when the pectoral girdle is lifted up. It and a small 

 muscle lateral to it (omohyoid) exert a pull upon the body of the hyoid, causing 

 it to bulge outward, and hence lower the floor of the buccal cavity in breathing. 

 Their action is thus the opposite of that of the preceding muscles. 



(5) The petrohyoids are several small muscles under the sterno- and omo- 

 hyoids extending from the otic capsule to the sides of the hyoid apparatus. 

 They raise the hyoid apparatus and hence the floor of the buccal cavity in 

 respiration, acting in antagonism to the preceding muscle, and by their com- 

 pressing effect upon the larynx and pharynx are of great importance in swallowing 

 food and air. 



/) Muscles of the tongue: 



(1) The hyoglossus is the conspicuous muscle in the median ventral line of 

 the throat under the geniohyoid. Each half of it originates at the posterior 

 end of the thyroid process of the hyoid, extends forward covering this process, 

 meets its fellow where the processes spring from the body of the hyoid. The 

 muscle thus formed runs forward in contact with the body of the hyoid up to the 

 base of the tongue into which it disappears. It is the retractor of the tongue 

 (draws it back into the mouth after use). 



(2) The genioglossus is a small but thick muscle lying in front of the anterior 

 end of the hyoglossus and originating from the lower jaw under the submental 

 muscle. It is the protractor of the tongue (throws it forward). 



g) Muscles of the thigh: The thigh presents apparent dorsal and ventral sides 

 and anterior and posterior surfaces. These are not really such because the leg 

 of the frog has been twisted from the primitive vertebrate position. The 

 ventral surface is really inner and hence more correctly called preaxial; and the 

 dorsal surface is outer or postaxial. Anterior is really ventral and posterior 

 dorsal. However, in order to simplify the following description, the same 

 nam^s will be applied as for other parts of the body, according to the apparent 

 positions. Separate all of the muscles of the thigh from each other before 

 proceeding. 



(1) The triceps femoris is the great muscle which covers the whole anterior 

 part of the thigh, its powerful tendon passing over the knee to the tibio-fibula. 

 It has three origins or heads and consists of three parts. The ventral part 

 (oastus internus or crural] arises from the borders of the acetabulum. The small 

 middle portion (tensor fasciae latae) originates on the ilium and ends in the 

 fascia (fascia lata), which covers the triceps femoris. The dorsal part is the 

 vastus externus or glutens magnus and arises from the side of the posterior end 

 of the crest of the ilium. The triceps femoris is the great extensor of the shank, 

 and also may draw the leg up against the body (abduction}. 



(2) The iliacus muscles are those which extend from a considerable part of 

 the crest of the ilium between the tensor fasciae latae and the vastus externus 

 to the femur. They are abductors of the leg. 



