60 



HANDBOOK OF PHYSIOLOGY. 



their greatest development where the Haversian systems are few, and 

 vice versa. 



The ultimate structure of the lamellae appears to be reticular. If a 

 thin film be peeled off the surface of a bone, from which the earthy 

 matter has been removed by acid, and examined with a high power of 

 the microscope, it will be found composed of- a finely reticular struc- 

 ture, formed apparently of very slender fibres decussating obliquely, but 

 coalescing at the points of intersection, as if here the fibres were fused 

 rather than woven together (fig. 63). 



In many places these reticular lamellae are perforated by tapering 

 fibres called the Claviculi of Gagliardi, or the perforating fibres of 

 Sharpey, resembling in character the .ordinary white or rarely the elastic 



Fig. 64. Lamellae torn off from a decalcified human parietal bone at some depth from the sur- 

 face, a, a, Lamellae, showing reticular fibres; 6, 6, darker part, where several lamellae are super- 

 posed; c, perforating fibres. Apertures through which perforating fibres had passed, are seen es- 

 pecially in the lower part, a, a, of the figure. (Allen Thomson.) 



fibrous tissue, which bolt the neighboring lamellae together, and may be 

 drawn out when the latter are torn asunder (fig. 64). These perforating 

 fibres originate from ingrowing processes of the periosteum, and in the 

 adult still retain their connection with it. 



Development of Bone. From the point of view of their develop- 

 ment, all bones may be subdivided into two classes. 



(a.) Those which are ossified directly or from the first in membrane 

 or fibrous tissue, e.g., the bones forming the vault of the skull, parietal, 

 frontal, and a certain portion of the occipital bones. 



(b.) Those whose form, previous to ossification, is laid down in hya- 

 line cartilage, e.g., humerus, femur. 



The process of development, pure and simple, may be best studied in 

 bones which are not preceded by cartilage, i.e., membrane- formed (e.g., 



