A FROG 193 



articulates dorsally with the last vertebra. The ventral portion 

 is disk-shaped and is composed of the ventral end of the ilium, a 

 small, triangular bone called the ischium, and a small, triangular 

 cartilage called the pubis ; the pubis is anterior to the ischium in 

 position. 



Carefully strip the muscles from the pelvic girdle, disarticulate 

 it from the vertebral column, and remove it and the hind legs 

 from the body. Disarticulate the legs and thoroughly clean the 

 pelvis. 



The Hind Leg. The skeleton of this leg closely corresponds to 

 that of the foreleg. It is made up of three divisions, a proximal, 

 a middle, and a distal division. 



The proximal division, or thigh, is composed of a single bone, 

 the femur, the head of which fits into the acetabulum and forms 

 the hip joint. 



The middle division, or shank, is composed of a single bone, the 

 tibiofibula. It is formed by the fusion of the tibia and the fibula, 

 the two bones which are present in the shank of most vertebrates. 

 The line of division between the two is very distinct. 



The distal division is composed of the tarsus, or ankle, and the 

 foot. The tarsal bones are five in number, arranged in two rows, 

 a proximal and a distal row. The proximal row consists of two 

 long bones, the astragalus and the calcaneum, which are united 

 at both ends. The latter is on the inner side of the foot and cor- 

 responds to the heel bone of the higher vertebrates. The distal 

 row is composed of three very small bones. The foot is made up 

 of six digits, of which one is supernumerary and rudimentary ; the 

 others are the five digits which characterize the typical vertebrate 

 foot. The supernumerary digit is on the inner side of the foot and 

 consists of from one to three small bones. Each of the other five 

 is composed of two parts : the metatarsus, a long bone which 

 articulates with the tarsus ; and the phalanges, two to four smaller 

 bones which form the toe. The first digit on the inner side of the 

 foot corresponds to the great toe. 



Exercise 25. Draw the ventral aspect of the leg on a scale of 2, show- 

 ing accurately the outlines of the bones and cartilages. 



