ENDOSKELETON AND VOLUNTARY MUSCLE 75 



vessels and some nervous tissue, which enter from the periosteum 

 covering the bone. The lacunae enclose bone cells from which 

 processes extend into the fine radiating canals. 



Skeletal Muscle. — If the skin is removed from the body of a 

 frog, the superficial skeletal muscles are exposed. Muscles are 

 usually attached by one or both ends to bones by tendons, which 

 are composed of white connective tissue fibers. Fasciae are 

 slightly elastic connective tissue membranes covering muscles, 

 and uniting with the tendons at the ends of the muscles. The 

 origin of a muscle is the end that remains stationary, or relatively 

 so, when the muscle contracts. The insertion is the end that 

 moves during contraction. Since movements are brought about 

 by the contractions of muscles, the flexion or extension of a body 

 part at a joint is the result of the actions of two sets of antago- 

 nistic muscles known as flexors and extensors, respectively. 

 Adduction or abduction refers to movements of the arms or legs 

 toward or away from the mid-line of the body, or in the case of 

 fingers or toes, toward or away from the principal axis of the arm 

 or leg. A rotator muscle causes a part to rotate about its main 

 axis. In connection with the jaws a levator muscle raises the 

 lower jaw and a depressor lowers it. The description of the 

 muscles of the bullfrog, Rana catesbeiana, in the following 

 paragraphs, is confined to those muscles that can be exposed 

 by the removal of the skin and fasciae and by a small amount of 

 dissection as illustrated in Figs. 47 and 48. 



Muscles of the Head and Trunk. — Beginning at the ventral 

 side of the head, the submaxillaris muscle, forming the floor of the 

 mouth, can be seen. Its fibers run transversely between the 

 rami of the lower jaw. It is important in swallowing and 

 respiratory movements. If one-half of the submaxillaris is 

 removed, three other muscles are exposed: (1) the submentalis, 

 a very small muscle joining the tips of the dentary bones that 

 aids in closing the external nostrils by drawing together the two 

 halves of the lower jaw and thus raising the premaxillary bones; 

 (2) the hyoglossus and (3) the geniohyoideus, two ribbon-shaped 

 muscles extending from the submentalis posteriorly. The outer 

 one, the geniohyoideus, arises from the lower jaw and is inserted 

 on the hyoid bone. The inner one, the hyoglossus, has its origin 

 in the hyoid bone, runs forward uniting with the muscle of the 

 opposite side and passes into the tongue at the anterior end of 



