850 



OSSEOUS TISSUE. 



have to be considered in relation to their num- 

 ber, their size, and the parts which they contain. 

 The number of canals in a given space is 

 perhaps a little variable, but this variation will 

 be regulated in some degree by the situation of 

 the bone, but more especially by the age of the 

 bone. Thus the transverse section of the femur 

 of a human foetus of seven months will present 

 many more canals than a section of equal mea- 

 surement from the femur of an adult. 



In certain fish of which the Scarus is a spe- 

 cimen, the Haversian canals are extremely nu- 

 merous, so that bone cells become unnecessary, 

 for here we find very few indeed, and in some 

 sections none, (fig- 451.) 



Fig. 451. 



Section of bone from the Scarus, showing that where 

 the Haversian canals are very numerous the bone 

 cells are absent. 



The size of the Haversian canals takes a con- 

 siderable range, varying from the 3 J 3 rd to the 

 sjLth of an inch, as stated by Mr. Smee. In the 

 young subject they seem larger than in the old. 

 But by far the most marked difference in size of 

 these canals is to be observed in the antlers of 

 the stag at different periods of their growth. 

 At an early period of the existence of the antler, 

 the vascular canals are large and numerous, 

 while at the time of their completion in size 

 the canals are less numerous in an equal 

 space, and very small : indeed many seem all 

 but obliterated. The density of bone is pro- 

 duced more by the small size of the canals than 

 by their comparative infrequency, though un- 

 doubtedly they are less frequent in the compact 

 bone, as that composing the shafts of long bones. 



In tracing individual canals, it will be found 

 that the majority maintain the same size as far 

 as we can follow them. This is not, however, 

 observable in all. If a large canal be taken 

 where it first enters the substance of the bone, 

 it may be found giving off branches from time 

 to time in various directions, and then again 

 sending off smaller branches, which anastomose 

 freely with each other, often joining at right 

 angles. 



Although it is very easy to trace a large 

 canal pervading a bone, and then dividing from 

 time to time into smaller ones, I have never 

 been able to satisfy myself that these small 

 canals again unite to form a second large canal, 

 and thus to leave the bone. I am therefore 

 led to the opinion that such does not occur, 

 but that the small even-sized canals open and 



give exit to their vessels upon the surface of the 

 bone generally, while the large canals give 

 entrance to arteries. 



The Haversian canals undoubtedly give pas- 

 sage to bloodvessels, which is their principal, 

 if not their only purpose. Whether they con- 

 tain one or more vessels seems to admit 

 of a little doubt. Dr. Carpenter, in his work 

 on Physiology, states that they contain an 

 artery and vein. From my own observation I 

 am not able to confirm his view. Indeed I am 

 disposed to the opinion that they give passage 

 to one vessel only ; that the larger canals 

 which are found entering the bone convey an 

 artery ; that it divides from time to time after the 

 manner of the canals described ; and that the 

 vessels emerge again from the surface of the 

 bone as capillaries. This branch of the subject 

 requires some further investigation. The fore- 

 going observations apply only to dense bone. 

 Where bone is cancellated for the reception of 

 fat, the vessels occupy but a small space in the 

 cancelli. 



Of the corpuscles or cells of bone ; also 

 called lacuna by Dr. Todd and Mr. Bowman. 

 The so-called corpuscles are nothing more 

 than small cells existing in the substance of the 

 tissue, and might with propriety be called bone 

 cells. Some anatomists have designated them 



Fig. 452. 







: 



,-.>. - fi^f^?^^ 



Section of a flat bone, showing the bone cells in the 

 granular tissue. 



calcigerous cells, from the supposition that 

 they contain in their interior an amorphous 

 salt of lime. That this view is incorrect will 

 be subsequently shown. The cells cannot be 

 described as having any definite unvarying 

 shape or size. The general form is a com- 

 pressed oval, though not unfrequently they are 

 circular, but flattened from side to side. Again, 

 they are sometimes almost triangular in their 

 outline, while in other instances they approach a 

 linear shape. These are the most common va- 

 rieties of outline to which the bone cells are sub- 

 ject ; as they occur in the bones of man and the 

 higher animals. But connected with the cells 

 are numerous delicate branching tubes, which are 

 slightly dilated as they enter the cells. The 

 number arising from each cell does not allow 



