278 SPECIAL HISTOLOGY. 



canal. If the section strike the surface of a system, the superficial 

 lacunas come into view, presenting very elegant forms, rounded or oval 

 (Figs. 115 d, and 117), surrounded in an irregular manner by a complete 

 tuft of canaliculi, which look directly towards the observer, and con- 

 sequently appear more or less shortened, and by a smaller number of 

 other canaliculi distributed on the surface of the lamellae. Occasion- 

 ally, even in the thinnest parts of a section, there occurs a tuft of canali- 

 culi, cut across transversely, and without the lacuna to which they 

 belong, whence these portions of bone exhibit a sievelike aspect. All 

 the canaliculi arising from the inner aspect of the innermost lacunae of 

 an Haversian system, proceed towards the canal, with which they, by 

 this means, communicate, as may be clearly seen in thin, perpendicular, 

 and transverse sections of bones filled with air, and in the walls of 

 medullary canals laid open longitudinally. From the borders and ex- 

 ternal aspect of the same lacunae other canaliculi are given off, which per- 

 haps occasionally terminate in blind extremities, but for the most part 

 communicate with those of the neighboring, and particularly of the outer 

 lacunae. The succeeding rows of lacunae are all mutually connected in a 

 similar way, and thus the network of canaliculi and lacunae extends to the 

 outermost lamellae of the system, where the lacunae either commuincate 

 with those of the contiguous systems or of the interstitial lamellae, or 

 terminate independently, in which latter case (Fig. 115 d) all the ca- 

 naliculi, or at least most, and the longest of them, proceed inwards, that 

 is to say, towards the vascular canal, from which they derive their 

 nutritive fluid. 



In the interstitial osseous substance between the Haversian systems, 

 when it exists in small quantity, the few lacunae, frequently not more 

 than from 1 to 3 in number, are disposed more irregularly, and also 

 present a rounded form (Fig. 115 e); when the interstitial substance is 

 more abundant, and distinctly lamellar, the lacunae are also disposed 

 more regularly, with their sides parallel to those of the lamellae. The 

 canaliculi of these lacunae, in like manner, communicate with each 

 other, and with those of the neighboring systems. In the outer and 

 inner fundamental lamellae, lastly, the lacunae are all placed with their 

 surfaces parallel with those of the lamellae, and consequently looking, 

 for the most part, inwards and outwards, or towards the centre and 

 periphery of the bone. In transverse sections they precisely resemble 

 those of the Haversian systems, only that they are but little or not at 

 all curved, except in the smallest cylindrical bones. In longitudinal 

 sections, whether perpendicular or parallel to the surface, they present 

 the conditions above described, with this limitation, however, that a 

 larger number of lacunae, of course, are seen in the same space in the 

 latter case than in the former, and also that the sievelike aspect described 

 above is more frequently observed, giving the bone considerable resem- 



