3G OSTEOLOGY. 



ments and muscles, and are usually well marked in proportion to the muscularity 

 of the subject. 



Microscopic Structure. If a thin transverse section from the shaft of a long 

 bone be examined with a power of about 20 diameters, a number of aper- 

 tures, surrounded by a series of concentric rings, are observed, with small, dark 

 spots grouped around them, also in a concentric manner. The apertures are 

 sections of the Haversian canals (so called after their discoverer, Clopton Havers); 

 the concentric rings are sections of the lamellae, which are developed around the 

 Haversian canals ; the dark spots are small cavities in the substance of the bone, 

 called lacunae. 



The Haversian canals are channelled out of the compact substance for the pur- 

 pose of conveying bloodvessels for its nutrition. They vary in size from the 

 3 ^ to the 2^'otr of an inch in diameter, the average size being about 5 ^Q. They 

 are generally round or oval, sometimes angular. Those nearest to the outer sur- 

 face, where the bone is most compact, are very small ; but, towards the medullary 

 canal, they gradually acquire a larger size, and open into it, or into the cells of tha 

 cancellous tissue. The Haversian canals are lined by a delicate membrane con- 

 tinuous with the periosteum; the smallest canals contain a single capillary vessel; 

 those larger in size contain a network of vessels : whilst the largest contain blood- 

 vessels and marrow. If a thin longitudinal section of the shaft of a long bone be 

 examined, the Haversian canals will be found to run in the long axis of the bone, 

 and parallel with each other, communicating freely by transverse or oblique canals, 

 so as to form, for the most part, rectangular meshes. Some of these canals open 

 on the outer surface, to admit bloodvessels from the periosteum ; others communi- 

 cate with the medullary canal, receiving bloodvessels from the interior of this part. 

 By this means, the Haversian canals establish a free communication between the 

 bloodvessels of the periosteum, and those of the medullary membrane. 



If a higher power is now applied to the same transverse section, each Haversian 

 canal appears surrounded by a series of concentric rings, varying in number from 

 eight to fifteen ; these rings are termed the lamellse, and their appearance is pro- 

 duced by transverse sections of concentric layers of bone that have been developed 

 around the Haversian canal, the last formed layer being deposited on that surface 

 next to the bloodvessel. This concentric arrangement is not complete around all 

 the canals; for here and there one set of lamellae may be seen ending between two 

 adjacent ones. Besides the lamellae surrounding the Haversian canals, some are 

 disposed parallel with the outer and inner surfaces of the bone ; these are termed 

 circumferential lamellse, and may be considered as concentric with the medullary 

 canal. Others, again, penetrate between the Haversian systems ; these are termed 

 interstitial lamellse. Each Haversian canal, together with its concentric lamellae 

 of bone, lacunae, etc., is called an Haversian system, the bloodvessel contained in 

 the central canal being the source of nutrition to the lamellae which surround it. 

 Nearly the whole of the compact tissue is made up of these Haversian systems, 

 each one being, to a certain extent, independent of the rest. In a longitudinal 

 section, the lamellae are seen running in lines parallel with the Haversian canal 

 which they surround, except when the section passes transversely or obliquely 

 across a canal, in which case an appearance is seen, somewhat similar to that 

 observed in a transverse section. This lamellated structure may be easily demon- 

 strated on a piece of bone softened in dilute acid, when the lamellae may be peeled 

 from the surface of the bone in a longitudinal direction. According to Dr. Sharpey, 

 the lamellae, in structure, consist of fine transparent fibres decussating each other, 

 so as to form a delicate network, the fibres apparently coalescing at their point 

 of junction. The lamellae are perforated, in certain situations, by bundles of 

 fibres which penetrate them in a more or less oblique direction, serving to securely 

 approximate the several plates. The lamellae are also perforated by numerous 

 minute apertures placed at regular distances apart, which are, probably, transverse 

 sections of the canaliculi. In this fibrous basis of the lamellae, the inorganic 

 elements of bone are intimately united. 



