18 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



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Bones are formed by ossification in two ways intracartilagi- 

 nous and intra membranous. 



The vertex of the skull i. c., the parietals, the frontal, the 

 tabular part of the occipital, the squamous, and tympanic 

 parts of the temporal, the inner plate of the pterygoid process, 

 the bones of the face except the inferior turbinate and part 

 of the lower jaw are formed in membrane. The base of the 

 skull and all other bones are formed in cartilage. A deposit 

 of bone begins at one spot, the primary centre; the shaft, or 

 diaphysis, is formed from this. Most bones have secondary 

 or tertiary centres of ossification, and parts derived from them 

 are the epiphyses (growths upon). The growth of bone in 

 length depends upon the cartilage between the epiphyses and 

 diaphysis; this cartilage acts as a buffer. Growth in circum- 

 ference is subperiosteal. 



All bones are covered with a vascular, connective-tissue 

 membrane, called periosteum. 



For the sake of description, bones are divided into four 

 classes: (1) Long, (2) short, (3) flat, and (4) irregular bones. 



Long bones present a shaft and two extremities. The 

 shaft is hollow and more or less cylindrical. The walls are 

 made of dense bone, thickest near the middle, and decreasing 

 in thickness toward the ends. The hollow in the centre is 

 the medullary canal, and is lined with internal periosteum, or 

 medullary membrane, and contains marrow in the recent state, 

 The extremities are expanded portions, made up of spongy 

 bone, closed in by a thin layer of compact bone. The inter- 

 stices of the spongy bone also contain marrow. Examples, 

 femora and humeri. 



Short bones are small, made up of spongy bone, with a 

 compact bony shell. Examples, carpals and tarsals. 



Flat bones have two compact plates enclosing a spongy 

 layer, the diploe. 



Irregular bones are such as cannot be classed with either 

 of the other groups. They are mostly situated symmetrically 

 across the median plane of the body. Their composition is 

 a compact shell enclosing spongy bone, which makes up the 

 great bulk of these bones. Examples, vertebrae, sphenoids, 

 malars, etc. 



Medullary Arteries. The medullary arteries of the large, 

 long bones of the extremities run from the knee and toward 



