"HI 



of movements at its articulation with the carpus. The metatarsal bone of the 

 great toe assists in supporting the weigh t of the body, is constructed with great 

 solidity, lies parallel with the other metatarsals, and has a very limited degree of 

 mobility. The carpus is small in proportion to the rest of the hand, is placed 

 in line with the forearm, and forms a transverse arch, the concavity of which 

 constitutes a bed for the Flexor tendons and the palmar vessels and nerves. The 

 tarsus forms a considerable part of the foot, and is placed at right angles to the 

 leg, a position which is almost peculiar to man, and has relation to his erect pos- 

 ture. In order to allow of their supporting the weight of the body with the least 

 expenditure of material the tarsus and a part of the metatarsus are constructed 

 in a series of arches (Figs. 290, 291), the disposition of which will be considered 

 after the articulations of the foot have been described. 



FIG. 291. Skeleton of foot. Lateral aspect. 



The Sesamoid Bones (Ossa Sesamoidea). 



Sesamoid bones are small more or less rounded masses embedded in certain 

 tendons and usually related to joint surfaces. Their functions probably are to 

 modify pressure, to diminish friction, and occasionally to alter the direction of a 

 muscle pull. That they are not developed to meet certain physical requirements 

 in the adult is evidenced by the fact that they are present as cartilaginous nodules 

 in the fetus, and in greater numbers than in the adult. They must be regarded, 

 according to Thilenius, as integral parts of the skeleton phylogenetically inherited. 1 

 Physical necessities probably come into play in selecting and in regulating the 

 degree of development of the original cartilaginous nodules. Nevertheless, irreg- 

 ular nodules of bone may appear as the result of intermittent pressure in certain 

 regions, e. g., the "rider's bone," which is occasionally developed in the Adductor 

 muscles of the thigh. 



Sesamoid bones are invested by the fibrous tissue of the tendons, except on the 

 surfaces in contact with the parts over which they glide, where they present 

 smooth articular facets. 



In the upper extremity the sesamoid bones of the joints are found only on the 

 palmar surface of the hand. Two, of which the medial is the the larger, are constant 

 at the metacarpophalangeal joint of the thumb; one is frequently present in the 

 corresponding joint of the little finger, and one (or two) in the same joint of the 

 index finger. Sesamoid bones are also found occasionally at the metacarpopha- 

 langeal joints of the middle and ring fingers, at the interphalangeal joint of the 

 thumb and at the distal interphalangeal joint of the index finger. 



In the lower extremity the largest sesamoid bone of the joints is the patella, 

 developed in the tendon of the Quadriceps femoris. On the plantar aspect of the 

 foot, two, of which the medial is the larger, are always present at the metatar- 



1 Morpholog. Arbeiten, 1906, v. 309. 



