322 



THE TISSUES. 



Haversian rod. In the latter case the lamina is seen to consist of 

 two distinct layers an external granular, and an internal, hyaline. 

 As a general rule, no particle of osseous tissue is found more 

 than T ^o of an inch from a bloodvessel. If, therefore, a plate of 

 bone be not more than double this thickness ( 5 J 5 of an inch), no 

 vessels will be found to enter it ; but they will be distributed upon 

 its two surfaces only. It is, however, indispensable for the nutri- 

 tion of bone that the plasma be brought more nearly than this to 

 each molecule of the osseous tissue ; and for this purpose cavities 

 of a peculiar form (lacunas and pores) are hollowed out in the sub- 

 stance of all but the thinnest laminae of bone, whose forms are 

 shown by Fig. 212. In some of the thinnest bones of the smallest 



Fig. 212. 



Two lacunje of osseous tissue seen on the surface, showing the disposition of their pores. The 

 ultimate granules of the tissue, both on their walls and around them, are well represented. (Magni- 

 fied 1,200 diameters.) From the cancelli of the femur. (Tomes.) 



animals, however as the os unguis of the mouse and certain small 

 birds even these cavities do not exist; every molecule of the 

 osseous tissue being so near to the blood in the vessels distributed 

 upon the surfaces of the bone, that no arrangement for the trans- 

 mission of the plasma alone, as before mentioned, is required. Such 

 a bone is therefore everywhere granular and homogeneous in struc- 

 ture, like the portions between the pores in Fig. 212. 



If, therefore, a layer of bone is not more than 3^3 to ^^ of 

 an inch thick, the substance is solid; if another layer be superim- 

 posed upon this, cavities for the plasma (lacunas and pores) are 

 formed between them, or in their substance; and if the whole 

 thickness be more than B ^ of an inch, vessels also (the Haversian 

 vessels) are found in canals traversing it. These cavities and canals 

 will be next described. 



