240 



MANUAL OF HISTOLOGY. 



named) or ossein permits of being cut up owing to its cartilaginous 

 consistence. 



In vertically cut plates of compact substance from long bones (fig. 



227) we may recognise the following 

 points. The whole is traversed by a 

 system of canals formed of longitudinal 

 passages connected in a reticular man- 

 ner with one another (a, b, c. d\ and 

 having an average diameter of 0*1 1 28- 

 0*0149 ram., with extremes on both 

 sides. These run more or less parallel 

 with one another, separated by in- 

 tervals of about 0-1128-0-2802 mm. 

 At certain intervals also connecting 

 tubes are seen passing between these at 

 one time directly transverse, at another 

 rather more obliquely. If the section 

 include the whole thickness of the 

 bone, some of these canals may be 

 observed to open freely into the 

 medullary cavity internally, as well as 

 externally towards the periosteum, 

 widening as they do so into funnel- 

 shaped orifices. 



Towards the ends of the long bones, 

 in the neighbourhood of their articular 

 cartilages, certain bends in the course of 

 the medullary canals may be observed. 

 This system of passages is destined for 

 the adm ission into the bone of the blood- 

 vessels necessary for its nutrition. The 

 passages themselves are known by the 

 nameofHaversian or medullary canals. 

 Transverse sections, as fig. 233, have 

 of course quite a different appearance. 

 Here we see, at corresponding dis- 

 tances, the severed ends of the longi- 

 tudinal canals in the form of rounded 

 apertures (c, c} ; or should the section 

 have been made somewhat obliquely, 



of more or less oval deficiencies of substance. Again, if the cut have 

 fallen in the plane of one of the transverse intercommunications between 

 two such canals, the latter appear as round holes connected by an open 

 slit. Intermediate forms occur also as a matter of course. 



This beautiful regularity, however, presented by the central portion of 

 a long bone, is more or less at an end in other than compact osseous 

 tissue. In the external crust of tabular bones, the Haver sian canals 

 generally run in a direction parallel with the surface; in most cases also 

 radiating from a central point. In the short bones also there is usually 

 one preponderating direction in their course. 



In the bands and septa of spongy osseous tissue, this system of 

 medullary canals is far less strongly developed, the latter frequently 

 opening into the cancellous spaces with funnel-shaped enlargements. 



Fig. 233. A portion of a human metacarpal 

 bone in transverse section, a, external, 6, 

 internal, surface with their respective gene- 

 ral lamellae ; c, transverse sections of Haver- 

 rian canals surrounded by their special 

 lamellae; d, intermediate lamellae; e, bone 

 corpuscles with their ramifications. 



