THE SPECIAL ANATOMY OF THE FROG 55 



b) Pubis, the anterior portion of the semicircular crest which projects 

 ventrally from that part of the pelvic girdle which lies medially between the 

 heads of the two long thigh bones. 



c) Ischium, the posterior portion of the crest. 



The two pubes and the two ischia are completely fused in the median ventral 

 line producing the projecting crest mentioned above. These unions are named 

 the pubic and ischial symphyses. For the exact boundaries between the three 

 bones of the girdle see Holmes, Fig. 69, p. 242. The cuplike cavity on each side 

 of the girdle which receives the head of the thigh bone is called the acetabulum. 



7. Bones of the limbs. The skeleton of the fore and hind limbs is evidently 

 built upon the same plan, and the bones evidently correspond. The two limbs 

 will therefore be considered together (Holmes, pp. 241, 243, and Fig. 63, p. 230). 



a) The upper part of each limb consists of a long bone. In the fore limb 

 this is the humerus; in the hind limb, the femur. The humerus bears a conspicu- 

 ous crest, the deltoid ridge, so named because the deltoid muscle is inserted there. 



b) The next section of the limb is generally composed of two bones, but in 

 the frog these two are in the case of both limbs fused into one. This is the radio- 

 ulna in the fore limb, tibio-fibula in the hind limb. A longitudinal groove along 

 the center of both surfaces of each of these bones indicates the place of fusion of 

 the two originally separate components, the radius and ulna in the forearm, the 

 tibia and fibula in the shank. Radius and tibia correspond and are on the thumb 

 side (preaxial side) of the limb; ulna and fibula are on the little finger side 

 (postaxial). 



c) Wrist and ankle constitute the next section of the limbs. The wrist or 

 carpus consists of six small bones in two rows. These are generally difficult to 

 make out in ordinary preparations of the skeleton. The ankle or tarsus is 

 unusually elongated in the frog and consists chiefly of two relatively large bones, 

 the astragalus on the preaxial side and the calcaneum on the postaxial side. 

 Between these and the foot, two or three minute bones occur. 



d) The palm of the hand and sole of the foot each consist of five slender 

 diverging bones, the metacarpals and metatarsals, respectively. The first meta- 

 carpal is rudimentary. 



e) The fingers and toes are supported by small bones, called phalanges. 



I. THE MUSCULAR SYSTEM 



The frog has two types of muscles, the involuntary muscles found in the 

 viscera, and the voluntary ones attached either directly or indirectly to the 

 skeleton. The arrangement of the muscles of the viscera has already been seen 

 in the study of the microscopic structure of the organs. They are commonly 

 arranged in cylindrical tubes, in which the fibers run in either a circular or a 

 longitudinal direction. The voluntary muscles, on the other hand, have no 



