SESAMOID BONES. EXTREMITIES OF THE FffiTUS. 221 



violent concussions, in consequence of falling upon other parts 

 of the body, who are free from these effects when they fall 

 upon the feet. 



The Sesamoid Bones 



Are seldom larger than half a pea. They are most commonly 

 found at the second joint of the thumb, and of the great toe ; 

 and are placed in pairs, especially at the great toe, between the 

 tendons of the flexor muscles and the bones.* In these situa- 

 tions they are convex externally, and on their internal surfaces 

 they are concave and covered with cartilage. 



They are also sometimes found between the heads of the 

 gastrocnemius muscle and the condyles of the os femoris. 



In the joints of the thumb and toe they appear to be very 

 analogous to the patella. 



Besides the four pair of sesamoid bones above described as 

 belonging to the skeleton, viz. two upon the metacarpo-pha- 

 langeal articulation of each thumb, and two upon the corres- 

 ponding joint of each great toe, there is often found in addition, 

 one upon the metacarpo-phalangeal joint of the little finger, 

 and upon the corresponding joint in the foot. There is one 

 also often met with in the tendon of the peroneus longus muscle, 

 where it glides through the groove in the cuboid bone. Some- 

 times they are found in the tendons that wind round the inner 

 and outer malleolus and in the psoas and iliacus where they 

 glide over the body of the os pubis. 



The Extremities of the Foetus. 



In the upper extremity the clavicle is almost perfect at birth ; 

 but the acromion and coronoid processes of the scapula, as well 

 as the head, are in a cartilaginous state. 



Both ends of the os humeri are cartilaginous. They after- 

 wards ossify in the form of epiphyses, and are united to the 

 body of the bone. 



The two bones of the forearm are in the same situation. 



* They are properly speaking developed in the tendons of these muscles, like 

 the patella in the tendon of the extensor muscles of the thigh. The patella 

 itself may be considered a specimen of a sesamoid, considering its mode of 

 development. p. 



19* 



