124 VERTEBRATE ZOOLOGY 



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 corre- 

 sponds to the heel bone. 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 

 p ar ts ? — 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 big toe. 



Exercise 31. Draw the ventral aspect of the leg on a scale of 2, show- 

 ing accurately the outlines of the bones and cartilages. 



The Axial Skeleton; the Vertebral Column. Strip the muscles 

 from the back. Disarticulate the head from the trunk. 



The vertebral column is composed of nine vertebrae and a long 

 unsegmented bone called the urostyle, which forms its posterior 

 portion. Four regions may be distinguished in it: a cervical 

 region, consisting of the first vertebra ; a thoracolumbar region, 

 consisting of the succeeding seven vertebrae ; a sacral region, con- 

 sisting of the last vertebra; and the urostyle, which represents 

 a caudal region. 



A vertebra is made up of the following parts : the centrum, or 

 body, which is the cylindrical, ventral portion ; the neural arch, on 

 the dorsal side of the centrum, which with it forms the neural 



