RAT, A REPRESENTATIVE MAMMAL 115 



closely fused together to form the innominate bone. The three bones 

 here are the long, anterior ilium, the ventral pubis, and the posterior 

 ischium. The socket into which the bone of the thigh fits is located 

 on the side of the innominate at the position where the three bones 

 unite. This socket is the acetabulum. Ventrally the two pubic bones 

 meet and fuse in the pubic symphysis. The bone of the thigh is the 

 femur, and it extends from the acetabulum, into which its head 

 articulates, to the knee. Besides the head at the proximal end there 

 are the greater and lesser trochanters. At the distal end the femur 

 is divided into medial and lateral condyles which articulate with the 

 bones of the shank. The small patella is the kneecap. The tibia, the 

 large bone of the shank, extends to the ankle. The slender arched bone 

 is the fibula, which articulates proximally with the lateral tuberosity 

 of the tibia and returns distally to fuse into the side of the tibia. 

 The ankle is composed of eight tarsals arranged in two rows. The 

 large bone of the proximal row is the calcaneum or heel. The arch 

 of the foot is supported by the five metatarsals, and the bones of 

 the digits (toes) are also phalanges. There are three in each except 

 the first toe, which has only two. 



The Muscular System 



Myology is the term applied to the study of muscles. The funda- 

 mental property of contractility in protoplasm is highly specialized 

 in muscular tissue. The function of muscular tissue, to produce 

 movement, is accomplished by shortening the muscle cells and later 

 relaxing them. Those muscles which are contracted under control 

 of the will are voluntary, and the others are involuntary. Most of 

 the skeletal (attached to the skeleton) muscles are voluntary, and the 

 visceral muscles of the internal organs are involuntary in their action. 

 The voluntary, skeletal muscles are the ones considered in this 

 chapter. This group functions primarily in locomotion. For the 

 most part each skeletal muscle is attached by one or both ends to 

 the periosteum (membranous covering) of bone, usually by tendons. 

 The origin of a skeletal muscle is the point of attachment on a fixed 

 bone or other part, which does not move as a result of the contrac- 

 tion of the muscle. The end of the muscle attached to the bone that 

 moves when the muscle contracts is the insertion. According to 

 function, muscles are classified as flexors, extensors, protractors, 



