38 THE INDIVIDUAL ORGANISM 



bones. The largest is the calcaneum, which forms the point of the heel; 

 the second largest is the astragalus, which lies above the heel bone and 

 articulates with the end of the tibia. In front of the tarsals are five longer 

 bones, forming the anterior part of the arch of the foot; these are the 

 metatarsals. The bones of the toes, like those of the fingers, are called 

 phalanges. There are two bones in the great toe and three in each of the 

 others, as with the thumb and fingers. 



The bones of the foot are so arranged as to form three arches that aid 

 in supporting the weight of the body. Two of these are the outer and 

 inner longitudinal arches; the third is the transverse arch. The highest 

 point of the longitudinal arches lies just in front of the ankle joint, in the 

 region of the tarsal bones. Although the bones of the foot provide the 

 chief structural elements of these arches, it is the tight binding of liga- 

 ments and muscles that enables them to yield elastically and yet keep 

 their arched shape when weight is placed upon them. The foot, in con- 

 formity with its weight-carrying function, is much stronger, more com- 

 pact, and less flexible than the hand. 



As may be seen by comparison, there is a high degree of correspondence 

 between the bones of the pectoral and pelvic girdles and limbs. This is 

 brought out in the following table. 



Pectoral Girdle Pelvic Girdle 



Scapula (shoulder blade) Ilium (hipbone) 



Pubis 

 Clavicle (collarbone) 



Coracoid process of scapula Ischium (on which we sit) 



Glenoid fossa (humeral socket) Acetabulum (femoral socket) 



Humerus (upper-arm bone) Femur (thigh bone) 



Radius (of forearm) Tibia (shinbone) 



Ulna (of forearm) Fibula (of shank) • 



Carpals (8) (wrist bones) Tarsals (7) (anklebones) 



Metacarpals (5) (palm bones) Metatarsals (5) (arch bones of foot) 



Phalanges (finger bones) Phalanges (toe bones) 



2 in thumb 2 in great toe 



3 in other fingers 3 in other toes 



Joints. We have seen that all the bones of the head are immovably 

 fused, except the lower jaw and the three pairs of tiny ear bones. The 

 bones of the hip girdle are also fused, and in the adult so are those of the 

 sacrum to which the hip girdle is attached. Most of these immovable 

 joints or sutures are tremendously strong, being reinforced by intricate 

 dovetailing along the lines of juncture. The first 10 pairs of ribs and the 

 collarbone are attached to the sternum by cartilaginous connections which 

 are more or less flexible. All the other joints between bones of the skeleton 

 are to some degree movable. Appropriate movements are provided for 



