n8 VERTEBRATE ZOOLOGY 



Kill a frog and completely remove the skin from the body, with- 

 out injuring any of the muscles. Inasmuch as they are more or 

 less transparent when fresh, it is well to let the animal lie in alcohol 

 or formalin before the muscles are studied. 



Fasten the frog on its back, with its head away from you, by 

 means of a pin through the tip of the nose and one through each 

 foot, and, without cutting anything, study the muscles of the ven- 

 tral side of the body. On the head the broad, thin submandibular 

 muscle, the fibers of which are transverse in direction, stretches 

 across from one side of the lower jaw to the other. A median 

 tendon separates the right half of it from the left half. A narrow 

 strand of muscle, the subhyoid, lies at the hinder end of the sub- 

 mandibular, with fibers parallel to its fibers and attached to it in 

 the medial area. The lateral ends of the subhyoid find their origin 

 in the hyoid cartilage. 



At the forward end of the trunk is a group of four muscles which 

 radiate from the base of the foreleg on each side toward the 

 medial area of the body and may be called the pectoral group. 

 Beneath the medial ends of these muscles between the forelegs will 

 be seen the delicate sternum, or breastbone. The foremost of these 

 muscles is the coraco-radial. It is a wide muscle, the hinder half 

 of which is concealed beneath the muscle next behind it. From its 

 distal end a long tendon passes along the humerus to the forearm. 

 The function of this muscle is to bend the forearm. 



The other three muscles of this group are divisions of the pec- 

 toral muscle. The two anterior divisions, like the coraco-radial, 

 have their origin in the breastbone near the median line. The 

 hindermost division, which is the largest, has its origin in the outer 

 edge of the broad aponeurosis which occupies the median area of 

 the abdomen. All the divisions of the pectoral muscle have their 

 insertion near the proximal end of the humerus, in the upper arm, 

 and serve to bend the arm back. 



Lying in front of the pectorals is the deltoid muscle. It consists 

 of two principal portions, an anterior and a posterior portion, both 

 of which arise near the median line along the forward border of 

 the pectorals. They are inserted in the humerus and cover the 

 insertions of the pectoral muscles. 



