OSTEOLOGY 7 



directly with the Haversian canal. The canaliculi thus constitute a 

 system of intercommunicating channels ■ which maintain a con- 

 nection directly and indirectly between a given Haversian canal 

 and the lacunae arranged concentrically around it, and traverse 

 the lamellse in their course. Each lacuna contains a protoplasmic 

 nucleated cell, called the hofie cell, which almost completely 

 fills it, and sends off processes into the canaliculi communicating 

 with it. The canaliculi contain nutritive fluid derived from the 

 arteriole. A given Haversian canal, with its concentric bone 

 lamella, concentric bone lacunae, and canalicuh, constitutes a 

 Haversian system, and compact bone is simply an aggregation of 

 such systems, with, in addition, the intermediate and circumfer- 

 ential lamellae. Certain lamellae are perforated perpendicularly by 

 fibres which thus bind them together. These fibres are known 

 as the perforating fibres of Sharpey. Some of them are composed 

 of white fibrous tissue, and others of elastic tissue, and those 

 nearest the periphery are connected with the periosteum from 

 which they are derived. Others, however, more deeply placed, 

 have no apparent direct connection with the periosteum. The 

 perforating fibres are absent from the lamellae of the Haversian 

 systems. 



Cancellated bone is composed of very slender trabeculae, which 

 are arranged in a reticular manner so as to enclose spaces, known 

 as the medullary spaces. These spaces, in the recent state, are 

 filled with marrow. The trabeculae consist of superimposed 

 lamellae of compact bone. The strongest lamellae are disposed in 

 the direction in which the greatest pressure has to be borne, and 

 these are known as -pressure lamella. Other lamellae which intersect 

 these, are spoken of as tension lamellce. The use of cancellated 

 bone is to impart sufficient strength without adding unduly to 

 weight. 



Classification of Bones. — Bones are arranged in four classes, as 

 follows: Long; short; tabular; and irregular. 



A long bone consists of a shaft and two articular extremities. 

 The shaft is more or less cylindrical, and contains a marrow canal, 

 which is surrounded principally by compact bone. The articular 

 extremities are composed of cancellated tissue, except at the sur- 

 face where there is a thin shell of compact bone. Long bones are 

 found in the appendicular skeleton. 



A short bone consists mainly of cancellated tissue, except at the 

 surface where there is a thin covering of compact bone. Short 

 bones are more or less oblong in shape, and are found in the carpus 

 and tarsus. 



A tabular bone is composed of two plates or tables of compact 

 bone, which enclose between them cancellated tissue. The scapula, 

 ilium, and tegmental bones of the skull belong to this class. In 

 the case of the latter the cancellated tissue is called diplo'e. 



An irregular bone is one which is so irregular in form and in the 

 relative distribution of the compact and cancellated tissues as to be 



