PRACTICAL ANATOMY. 



supplied with nutrition through branches of blood-vessels 

 which ramify in the periosteum. They enter the bone 

 through minute foramina on its surface, and run in the 

 Haversian canals. They are especially abundant about 

 the articular extremities, where the foramina are numerous 

 and large. The veins which run in the bone are, as a 

 rule, attached to the sides of the canals, and remain 

 patulous on section. This is particularly observed on 

 section of the cranial bones. The nutrient artery of a 

 bone does not supply nutrition to the bone itself, but 

 enters the medullary cavity, to the contents of which it 

 is distributed. 



The periosteum is a fibrous membrane which closely 

 invests the bone except at the articular surfaces. It 

 consists of two layers, an outer areolar and an inner 

 dense layer. In young life it is pinkish in color, thick 

 and vascular, but, as age advances, it becomes yellowish, 

 less vascular, quite thin, and adheres very tightly to the 

 bone. At the points of the insertion of tendons the 

 fibres of the periosteum blend with those of the tendon. 

 The periosteum is not only the investing vascular mem- 

 brane of the bone, but also a bone organ. In the grow- 

 ing bone its under surface is soft and granular and con- 

 tains an abundance of osteoblasts, active in increasing 

 the diameter of the bone ; nerves are distributed to it 

 freely. 



DEVELOPMENT AND GROWTH OF BONE. 



An examination of the skeleton of an embryo shows 

 that certain bones, such as those of the long and short 

 varieties, consist primarily of cartilage, while others, 

 such as the flat bones of the skull, are developed between 

 two membranes, without a pre-existing cartilaginous 

 mold. Hence we speak of two forms of ossification, 

 intra-cartilaginous and inter-membranous. As the 



