THE SKELETAL SYSTEM 



257 



Beginning with the ninth week, the coxal bone ossifies from three 

 centers corresponding with ilium, pubis, and ischium. In the region of 

 the acetabulum, ossification is not completed before the tenth year, and 

 ossification of the coxal bone continues until the twenty-fifth year. The 

 bones of the leg and foot develop in a manner resembling the manner of 

 growth of bones of the arm and hand. The bones are mostly long bones 

 and develop from the usual three centers, a diaphysis and two epiphyses. 

 Separate epiphyses are formed for the head of the femur and the greater 

 and lesser trochanters. Ossification of the patella begins in the third year. 



The tarsal bones begin to ossify before birth, but the epiphysis of the 

 calcaneus not until the ninth year. Metatarsals and phalanges begin 

 to ossify in the third month. 



Fig. 214. — Diagrams illustrating the formation of a joint (diarthrosis). A single 

 or double cavity may be formed. In the latter case the intermediate cartilage dis- 

 appears later. (Redrawn from Coming's "Human Embryology", after Testut.) 



Malformations of the hand and foot are not uncommon. Most of 

 them result from a duplication or a fusion of parts. They include fusion 

 of digits, extra digits, and even duplication of entire hands or feet. 



The Development of Joints. While the precartilaginous anlage of the 

 skeleton is usually described as a histological continuum, the statement 

 needs qualification. For in the regions where joints appear, the connective 

 tissue does not assume the characteristics of cartilage. Such undifferen- 

 tiated tissue resembles histologically, and is continuous with, the peri- 

 chondral sheath which surrounds each embryonic cartilage. When such 

 a joint persists throughout life without motion, the condition is known as 

 synarthrosis, and the connective tissue of the joint may change little 

 throughout life. With increasing age, however, such immovable joints 

 tend, as in the case of the sutures of the cranium, to close up or ankylose, 

 so that the bone becomes a continuum. 



When, however, a movable joint is formed between cartilages such as 

 those which form the phalanges, the connective tissue of the joint first 

 becomes gelatinous and then disappears leaving a liquid-filled cavity 

 or cleft between the phalanges. The connective tissue which surrounds 

 the joint, and which is continuous with the perichondrium of the phalanges, 

 persists and becomes differentiated as the ligamentous capsule of the 

 joint. Most of each phalanx is eventually converted into bone, but the 

 articulating surface of the joint remains cartilaginous throughout life. 



