112 LOCOMOTION. [CHAP. v. 



nourishment from the periosteum, and which may be called the 

 periosteal layer. The lacunae of this layer, all face that surface, and 

 the pores of the superficial ones open upon it. There is another 

 layer, forming the immediate wall of the medullary cavity, and 

 termed the medullary layer. Its lacunae, in like manner, face this 

 cavity; and the pores of the inner ones open upon it. This layer 

 becomes variously folded to form the plates and fibres of the can- 

 celli ; and all the lacunae of these face these irregular cavities, and 

 their pores open into them. The Haversian surface, too, being an 

 involution of the outer and inner surfaces, and serving to connect 

 them, is, in fact, formed by an involution of the periosteal and 

 medullary layers, and unites these with one another. Where a 

 vessel enters the compact tissue from the exterior, it carries with it 

 a sheath of bone from the periosteal layer. The lacunae of this 

 osseous sheath, instead of being turned outwards, like those of the 

 periosteal layer, preserve their relation to the vascular surface to 

 which they pertain, and face inwards towards the vessel. Where- 

 ever the vessel penetrates, whatever direction it takes, and however 

 it branches, it is everywhere accompanied by this sheath from the 

 periosteal layer, or by offsets from it; and, when it enters the 

 medullary canal, its sheath expands into the medullary layer. 



The vessels of the compact tissue are so close together that the 



osseous sheaths respectively surrounding them come into contact 



and unite ; and thus all the space between the outer and the inner 



surface of the compact tissue is filled up : thus, in a word, the com- 



fig ,25. P ac t tissue is constructed. 



As the vessels of the compact tissue take a 

 longitudinal direction, a transverse section of 

 the bone (fig. 25) will appear pierced by 

 numerous holes, which are the Haversian 

 canals cut across. Each hole appears as the 

 centre of a roundish area, which is the sec- 

 tion of an involuted periosteal layer now 

 become a vertical rod, containing a vessel in 

 its axis. The Haversian canals vary consi- 

 derably in size, and do not maintain a very 

 close relation to the thickness of their re- 

 spective osseous walls. They are frequently 



Transverse section of the com- ' , . ,*, . 11 v i 



pact tissue of a long bone; shew- eccentric, owing to their wall bulging more 



ing, a. The periosteal layer, b. -\- , .-t i r> 



The medullary layer, and the in- m OU6 direction than another, to fit in 



termediate Haversian systems of , ,1 , -i , / ,-1 



lamellae, each perforated by an tW66n Others in the VICimty : tor tnOUgJ 



about 15 the rods of bone, containing the vessels, affe 



