272 SPECIAL HISTOLOGY. 



canals have thin walls, those of a middle size thick ones, and the most 

 minute, again, walls of little thickness. The thinnest walls I have 

 commonly noticed measure 0-008-0-02, and the thickest, 0-08-0-1 of 

 a line. The thickness of the lamellae varies between 0-002 and 0*005 

 of a line, being on the average 0-003 to 0-004 of a line ; in number 

 there are usually from eight to fifteen ; sometimes, however, no more 

 than four or five, and occasionally as many as from eighteen to twenty- 

 two. 



The lamellae of the Haversian canals, together with their canals, 

 extend to the internal and external surfaces of the diaphyses, where 

 they are connected with the general lamellae above mentioned, the 

 fundamental lamellce (Fig. 111). The latter constitute an external and 

 an internal layer, and penetrate also into the substance of the diapliysis, 

 where they are interposed between the separate lamellar systems and 

 the medullary canals. The two former layers, or the external and in- 

 ternal fundamental lamella*, are parallel to the external and internal 

 surfaces of the bone, and vary in thickness apparently without any 

 definite rule, from 0-02 to 0-3, or even 0-4 of a line. The latter, or 

 interstitial fundamental lamella* , are seen most clearly where the super- 

 ficial fundamental laminae are developed, in partial connection and 

 parallel with which they extend from without inwards, and from within 

 outwards, some distance into the substance of the diapJiyses, where they 

 are interposed, in masses varying in thickness from 0-02 to 0-12 of a 

 line, between the other lamellae (Fig. 112 d). In the interior of the 

 compact substance, on the other hand, in man, the Haversian systems 

 are so closely crowded that there can be no question as to the non-exis- 

 tence of lamellar groups between them, and it is evident that those 

 lamellae, which in a transverse section appear in man to be parallel 

 with the surface, almost all belong to horizontal canals ; and it is but 

 rarely that distinct interstitial masses are seen, as is usually the case in 

 other mammalia. The thickness of the separate lamellae just described 

 is much the same as that of the lamellae of the Haversian canals, and 

 their number varies from 10 to 100. 



We have hitherto considered only the diapliyses of the long bones. 

 In their apophyses, the thin cortical layer of compact substance natu- 

 rally presents only a few systems of Haversian canals, which, however, 

 are constituted as elsewhere. The exterior fundamental lamellae are 

 few in number, and internally, owing to the existence there of the spongy 

 substance, they are wholly wanting. In the latter substance, the very 

 few Haversian canals present lamellar systems as usual, except that 

 they are thin, and the remainder, according to the condition of the 

 osseous network, consists of a lamellated and fibrous tissue, which in 



outermost of the lamina of an Ha versian^ canal (see Fig. 113) results from its being the first 

 deposition within the pre-formed irregular Haversian space. (Tomes and De Morgan, 1. c., 

 p. 5.) TRS.] 



