82 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 104. — Skull of a new-born child from the side (f). 



Fig. 105. — Skull of a new-born child from above (f). 



Fig. 106. — Skull of a new-born child from behind and below (f). 



3. The two sphenoidal jontanelles (Fig. 104) are of medium size, irregular in form, poorly 

 defined, and situated between the parietal angles of the greater wings of the sphenoid bones 

 and the sphenoidal angles of the parietal bones, in the location of the later-developed spheno- 

 parietal sutures and the contiguous bony margins. 



4. The two mastoid jontanelles (Fig. 106) are situated between the mastoid angles of the 

 parietal bones and the parietal notches of the temporal bones, extending outward to the neigh- 

 boring bony margins. They resemble the antero-lateral fontanelles in respect to their size, 

 shape, and boundaries. 



By the progressive ossification of the flat bones of the cranial vault, the fontanelles become 

 closed in the first year of life, rarely later, the frontal fontanelle closing last (at the end of the 

 first or the beginning of the second year). At the same time the coronal, sagittal, and lambdoid 

 sutures develop, whereby small bony areas frequently remain as independent structures within 

 the sutures, and are known as supernumerary bones, Wormian bones, or ossa suturarum. They 

 are particularly common in the sagittal and lambdoid sutures, where they are sometimes 

 present in large numbers and are occasionally of considerable size. 



The Skeleton of the Extremities. 



The skeletons of the upper and lower extremities are more or less similar. They are com- 

 posed of: (1) The girdle of the extremity, and (2) the free extremity. The shoulder girdle 

 is composed of the scapula and the clavicle; the pelvic girdle of the two pelvic bones. The 

 skeleton of each free extremity consists of a proximal, a middle, and a distal segment, these 

 being represented in the upper extremity by the bone of the arm, the bones of the forearm, and 

 the bones of the hand, and in the lower extremity by the thigh-bone, the bones of the leg, and 

 the bones of the foot. The skeleton of the proximal segment of each extremity consists of a 

 single bone: in the upper extremity, the humerus; in the lower one, the femur. The middle 

 segment is formed by two bones: the radius and ulna in the forearm; the tibia and fibula in 

 the leg. The distal segments, the hand and the foot, contain a number of bones, those of the 

 hand being subdivided into the carpal bones, the metacarpal bones, and the phalanges of the 

 fingers, and those of the foot into the tarsal bones, the metatarsal bones, and the phalanges of 

 the toes. 



The extremities also contain a number of sesamoid bones; they occur in the upper ex- 

 tremity only in the hand; in the lower extremity they are to be found both in the foot and also 

 in the region of the knee (the knee-cap or patella). 



