SESAMOID BONES. 259 



and one for the base, in the metatarsal bone of the great toe. Ossification com- 

 mences in the centre of the shaft about the seventh week, and extends towards 

 either extremity, and in the digital epiphyses about the third year ; they become 

 joined between the eighteenth and twentieth years. 



The Phalanges are developed by two centres for each bone ; one for the shaft, 

 and one for the metatarsal extremity. 



SESAMOID BONES. 



These are small rounded masses, cartilaginous in early life, osseous in the 

 adult, which are developed in those tendons which exert a great amount of pres- 

 sure upon the parts over which they glide. It is said that they are more com- 

 monly found in the male than in the female, and in persons of an active muscular 

 habit than in those who are weak and debilitated. They are invested throughout 

 their whole surface by the fibrous tissue of the tendon in which they are found, 

 excepting upon that side which lies in contact with the part over which they 

 play, where they present a free articular facet. They may be divided into two 

 kinds : those which glide over the articular surfaces of joints, and those which 

 play over the cartilaginous facets found on the surfaces of certain bones. 



The sesamoid bones of the joints are, in the lower extremity, the patella, 

 which is developed in the tendon of the Quadriceps Extensor; two small sesa- 

 moid bones, found in the tendons of the Flexor Brevis Pollicis, opposite the 

 metatarso-phalangeal joint of the great toe, and occasionally one in the meta- 

 tarso-phalangeal joint of the second toe, the little toe, and, still more rarely, the 

 third and fourth toes. 



In the upper extremity, there are two on the palmar surface of the metacarpo- 

 phalangeal joint in the thumb, developed in the tendons of the Flexor Brevis 

 Pollicis, occasionally one or two opposite the metacarpo-phalangeal articulations 

 of the fore and little fingers, and, still more rarely, one opposite the same joints 

 .of the third and fourth fingers. 



Those found in the tendons which glide over certain bones, occupy the follow- 

 ing situations: one in the tendon of the Peroneus Longus, where it glides through 

 the groove in the cuboid bone ; one appears late in life in the tendon of the 

 Tibialis Anticus, opposite the smooth facet on the internal cuneiform bone; one 

 is found in the tendon of the Tibialis Posticus, opposite the inner side of the 

 astragalus ; one in the outer head of the Gastrocnemius, behind the outer con- 

 dyle of the femur ; and one in the Psoas and Iliacus, where they glide over the 

 body of the pubes. Sesamoid bones are found occasionally in the tendon of 

 the Biceps, opposite the tuberosity of the radius; in the tendon of the Glutens 

 Maximus, as it passes over the great trochanter ; and in the tendons which wind 

 round the inner and outer malleoli. 



The author has to acknowledge valuable aid derived from the perusal of the works of Cloquet, 

 Cruveilhier, Bourgery, and Boyer, especially of the latter. Reference has also been made to the 

 following: "Outlines of Human Osteology," by F. 0. Ward. "A Treatise on the Human 

 Skeleton, and Observations on the Limbs of Vertebrate Animals," by G. M. Humphry. Hold- 

 en's " Human Osteology." Henle's " Handbuch der systematischen Anatomie des Menschen. 

 Erster Band. Brste Abtheilung. Knochenlehre." "Osteological Memoirs (The Clavicle)," by 

 Struthers. "On the Archetype and Homologies of the Vertebrate Skeleton," and "On the 

 Nature of Limbs," by Owen. Todd and Bowman's "Physiological Anatomy," and Kolliker's 

 ' Manual of Human Microscopic Anatomy," contain the most complete account of the structure 

 and development of bone. The development of the bones is minutely described in " Quain's 

 Anatomy," edited by Sharpey and Ellis. On the chemical analysis of bone, refer to " Lehmann's 

 Physiological Chemistry," translated by Day; vol. iii. p. 12. "Simon's Chemistry," translated 

 by Day ; vol. ii. p. 396. A paper by Dr. Stark, " On the Chemical Constitution of the Bones of 

 the Vertebrated Animals" (Edinburgh Medical and Surgical Journal; vol. liii. p. 308) ; and Dr. 

 Owen Rees' paper in the 21st vol. of the Medico-Chirurgical Transactions. 



