146 HUMAN BIOLOGY 



Attached to the spinal column are twelve pairs of ribs, ten pairs 

 of which connect with the breastbone and thus help to inclose the 

 chest cavity (Fig. 44). The arms are attached to the rest of the 

 skeleton by a movable girdle of bones consisting of the two shoulder 

 blades and the two collar bones. A complete and rigid circle of bones 

 is formed at the posterior end of the trunk by the two pelvic bones, 

 which are attached dorsally to the spinal column and meet in front. 

 On the outer side of each pelvic bone is a deep socket into which fits 

 the upper end of the thigh bone (Fig. 44) . 



211. Skeleton of the arm. The skeleton of the upper arm (see 

 Fig. 44) is formed by a single long bone called the hu'me-rus, which 

 extends from the shoulder to the elbow. In the forearm, one can 

 feel through the flesh two separate long bones, of about the same 

 size, lying side by side ; the bone on the thumb side of the forearm 

 is the ra'di-us; on the little finger side is the ul'na. Eight small 

 bones are found in the wrist ; and in the palm of the hand and in 

 the fingers are nineteen somewhat elongated bones. All these 

 twenty-seven bones move freely upon each other and thus give the 

 hand a great freedom of movement. 



212. Skeleton of the leg. In the upper part of the leg (see Fig. 

 44) is a single bone, the thigh bone or fe'mur. This corresponds in po- 

 sition to the humerus of the arm, but it is longer and stouter than 

 the latter ; in fact, it is the longest bone in the body. The skeleton 

 in the calf of the leg consists of two bones (tib'i-a and fib'u-la) 

 which have a position similar to that of the radius and ulna. The 

 tibia is on the inner or great-toe side and is much larger than the 

 slender fibula. At the knee joint one can feel a flat piece of bone, 

 more or less circular in outline, called the kneecap. The twenty-six 

 bones of the ankle and foot are in the form of an arch, one end of 

 which rests upon the heel. 



213. Skeleton of the head. Two groups of bones may be dis- 

 tinguished in the skull or skeleton of the head ; namely, the bones 

 forming the cranium, which surrounds and protects the brain, and 

 the bones that form the skeleton of the face (Fig. 44). 



