A TURTLE 151 



Exercise 28. Make a sketch of the anterior extremity, showing accu- 

 rately the outlines of all these bones. 



The Posterior Extremities. These are composed of the pelvic 

 girdle and the hind limbs. The skeleton of the former is composed 

 of three bones on each side, the dorsal ilium, and the ventral pubis 

 and ischium, the former of which is anterior to the latter. At the 

 point of meeting of these three bones is the acetabulum, the depres- 

 sion in which the femur articulates. The dorsal end of the ilium 

 articulates with the sacrum. The two pubic bones are joined in 

 the median line by a narrow cartilaginous plate. The two ischial 

 bones are smaller than the pubes and contiguous to them, and 

 are also joined by a median cartilage. 



The skeleton of the hind leg 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. The middle division, or shank, 

 contains two bones, the tibia and fibula, the former being some- 

 what larger than the latter and preaxial in position. The distal 

 division is composed of the ankle and the foot. The ankle, or 

 tarsus, contains five or six small bones in two rows, the proximal 

 row consisting of the smaller fibular and the larger astragalus ,— 

 which in some turtles are united to form a single bone,— and 

 the distal row of four tarsal bones, of which the largest has been 

 formed by the fusion of the original fourth and fifth tarsals. The 

 foot is composed of five digits, each of which contains a proximal 

 metatarsal bone and several phalanges. 



Exercise 29. Make a sketch of the posterior extremity, showing accu- 

 rately the outlines of all these bones. 



A fresh animal will be needed for the study of the brain and the 

 blood vessels. Kill the animal and remove the plastron, as di- 

 rected on page 133. Very carefully chip away a small part of the 

 wall of the brain case so as to admit fluids into it. The brain can 

 be studied to the greatest advantage after it has been hardened 

 in strong alcohol or in formalin. We shall, consequently, study 

 the blood vessels first in this specimen, and allow the brain to be 

 hardening while they are being dissected. 



