1*70 DEVELOPMENT OF BONE. 



enter the bone from the periosteum, by the numerous 

 foramina upon its surface. 2d. Those which enter by the 

 larger foramina at the extremities of the long bones, and 

 at difterent points upon the surface of others ; while the 

 3d, called nutritious arteries, enter the long bones by a sin- 

 gle foramen, and that near the centre of each. 



The arteries of the first two classes ramify minutely 

 throughout the whole of the compact and cellular structure. 

 The nutritious artery is much larger, and passes single 

 through the compact structure to the medullary cavity, 

 where it divides into two branches, an ascending and de- 

 scending, which ramify upon the medullary membrane in 

 countless capillary vessels, and anastomose freely with the 

 other two classes. 



The veins are numerous, those accompanying the nutri- 

 tious artery pass out of the same foramen, and return the 

 blood from the medullary membrane. The veins which 

 receive the blood from the other two classes of arteries do 

 not attend them, but pass out by numerous distinct open- 

 ings found upon the surface of the bones, and after a short 

 course join the general circulation. 



Lymphatics have been demonstrated in the medulla, but 

 not with certainty in bone. 



Nerves have been traced along the nutritious arteries, 

 but not into the substance of the bone itself 



DEVELOPMENT OF BONE. 



" In the human foetus and other animals, before the time 

 of birth," says John Bell, " instead of bones there are only 

 cartilages of the form of the future bone. The whole 

 foetus appears to the eye like a mere jelly. The bones are 

 a pure, almost transparent and tremulous jelly; they are 

 flexible, so that a long bone can be bent into a complete 

 ring, and no opacity or spot of ossification is seen." 



The development of bone consists of three stages 



1. The Mucous. 2. The Cartilaginous. 3. The Osseous. 



The mucous stage presents bone, like all the other tis- 

 sues, in the earliest period of the embyro, as one homoge- 



