176 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



294. In the adult bone, the cells which fill the remaining cavity of 

 these canals secrete fatty matter. This is particularly evident in the 

 case of the central cavity, which may be considered as an immensely 

 dilated Haversian canal, where they constitute the medulla or marrow. 

 It does not appear that these take any active part in the nutrition of 

 the bone ; indeed, in the bones of Birds the shaft is entirely hollow, 

 and air is admitted into it from the lungs, so that its lining membrane 

 is rendered subservient to the aeration of the blood. 



295. The arrangement of the elementary parts of the osseous sub- 

 stance around the Haversian canals is very interesting and beautiful. 

 When a transverse section of a long bone is made, the open orifices of 

 the longitudinal canals present themselves at intervals, sometimes con- 

 nected by a transverse canal where the section happens to traverse this. 

 Around these orifices, we see the osseous matter arranged in the form 

 of cylinders, which appear to be marked by concentric circles (Fig. 49, 

 1, 2). Now when one of these circles is minutely examined, it. is found 

 to be made up of a series of lacunae, analogous to those already 

 described ; these, however, are seldom or never so continuous as to form 

 a complete circle. The long sides of the lacunae are directed, the one 



Fig. 49. 



Minute structure of bone, drawn with the microscope from nature. Magnified 300 diameters. 1. One of 

 the Haversian canals surrounded by its concentric lamellae. The lacunae are seen between the lamellae 

 but the radiating tubuli are omitted. 2. An Haversian canal with its concentric laminae, lacunae, and 

 radiating tubuli. 3. The area of one of the canals. 4, 4. Direction of the lamellae of the great medullary 

 canal. Between the lamellae at the upper part of the figure, several very long lacunae with their tubuli are 

 seen. In the lower part of the figure the outlines of three other canals are given, in order to show their 

 form and mode of arrangement in the entire bone. 



towards the Haversian canal (3) in the centre, the other towards the 

 circular row next beyond it. And when the course of the canaliculi is 

 traced, it is found that these converge on the inner side towards the 

 central canal, inosculating with those of the series next within, whilst 

 those of the outer side pass outwards in a radiating or diverging direc- 

 tion, to inosculate with those of the series next external. Thus a com- 

 plete communication is formed, by means of this system of radiating 

 canaliculi and intervening lacunae, between the central canal and the 



