A FROG 187 



hinder end of the dorsal aorta the small median posterior mesen- 

 teric artery leaves its ventral surface and runs to the rectum. 



The common iliac arteries, which are formed by the division 

 of the dorsal aorta, are large vessels which supply the hind 

 legs. A short distance from its origin each gives off two small 

 arteries : the hypogastric, which supplies the rectum, bladder, 

 and ventral body wall ; and the femoral, which goes to the thigh. 

 After giving off these arteries the common iliac is known as the 

 sciatic artery. Follow it and its branches into the leg. 



Exercise 19. Draw a semidiagrammatic sketch showing these arteries 

 so far as they have been observed. 



Exercise 20. Draw a diagram of the entire arterial system. 



The Muscular System. Most muscles in the land vertebrates 

 are attached at both ends by means of tendons. One end 

 is usually attached to a more or less fixed and the other to a 

 more movable portion of the body, the former being called the 

 origin of the muscle and the latter its insertion. The middle part 

 of the muscle is called the belly ; by its contraction the origin 

 and insertion, and with them the skeletal pieces to which they 

 are attached, are brought nearer together. Muscles are usually 

 attached to the bones and cartilages ; thick, fibrous membranes, 

 called aponeuroses, which often cover muscles and other organs, 

 may, however, serve the same purpose. 



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

 without 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 

 ventral side of the body. On the head the broad, thin subman- 

 dibular 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 submandibular, with fibers parallel to its fibers and at- 



