BOXE. xcvii 



rarely found, and when present are smaller, in the concentric systems of Haversian 

 lamellae ; in this case they must of course have been formed from the vascular tissue 

 (similar in nature to that under the periosteum) which occupied the Haversian 

 spaces and produced the concentric laminae. Perforating fibres exist abundantly in 

 the crusta petrosa of the teeth. 



Fig. XLYII. 





vw^*^ 



Fig. XL VII. LAMELLA TORN OFF FROM A DECALCIFIED HUMAN PARIETAL BONE AT 



SOME DEPTH FROM THE SURFACE. 



, a lamella, showing reticular fibres ; 6, b, darker part, where several lamellse are super- 

 posed ; c, c, perforating fibres. Apertures through which perforating fibres had passed, 

 are seen especially in the lower part, a, a, of the figure. Magnitude as seen under a 

 power of 200, but not drawn to a scale (from a drawing by Dr. Allen Thomson). 



The perforating fibres, or rather bundles of fibres, for the most part agree in 

 character with the white fibrous tissue, but some, according to H. Muller, are of 

 the nature of elastic tissue. H. Muller has shown that in some parts the fibres 

 escape calcification, and thus, as they shrink in drying, leave tubes or channels 

 in the dry bone, generally leading from the surface inwardly. In this way he 

 explains the nature and mode of production of the " tubes " described by Tomes and 

 De Morgan as penetrating the bone in certain situations, and conjectured by them 

 to be modified lacunae.* I at one time believed that these tubes had no relation to 

 the perforating fibres, but I have no doubt of the correctness of Miiller's explanation ; 

 at the same time I am satisfied that uncalcified fibres, though numerous at particular 

 spots, are by no means so frequent as might be inferred from Miiller's account of 

 them, and that the perforating fibres may be said to be generally calcified. Finally, 

 these fibres seem to have no physiological significance : they may be regarded as 

 merely a modification of the mechanical structure of the tissue. 



In a thin transverse section of hard bone, the curved lines, or rather bands which 

 represent the cut edges of the lamellae, generally present, with transmitted light, a dark 

 granular-like, and a light, transparent, and usually narrower zone. Under a high 

 power of the microscope the former appears thickly dotted over with fine dark points. 

 In a decalcified section the dark part shows a multitude of short bright lines running 

 radially across it, with dark angular particles between them. The lines are probably 

 caused by pores and fine clefts passing through the lamella ; the appearance of dark 

 particles seems to me to be produced by the cut ends of the reticulating fibres of 

 which it is made up. A longitudinal section of a cylindrical bone carried across the 



* Phil. Trans. 1853, p. 116. 



