57 



difference in the measurement of the bone-cells, in consequence of 

 their long diameter being seen in the one case, and their short 

 diameter in the other ; and hence the caution of having all the sec- 

 tions made in one direction. In all doubtful cases the better plan is 

 to examine a number of fragments, both transverse and longitudinal, 

 taken from the same bone, and to form an opinion from the shape of 

 bone-cell Which most commonly prevails. 



If we examine minutely the excellent table of the comparative 

 sizes of the blood-particles in the four great classes of animals, drawn 

 up with much care and attention by Mr. Gulliver, we shall find that, 

 with very few exceptions, the blood-particles of the human subject 

 are much larger than those of birds measured in the short diameter, 

 and that those of reptiles are much larger than either of the two pre- 

 ceding ; and I have already mentioned that the bone-cells are the 

 largest in the reptiles, the next largest in the mammal, and the 

 smallest in the bird : now it would indeed be a curious result, if it 

 should ultimately turn out that the bone-cells of an animal are always 

 in proportion to the size of the blood-discs : my investigations at 

 present are not yet in a sufficiently forward state to warrant my 

 coming to such a conclusion, but still there are many cases in which 

 it holds good. It is well known to anatomists, that in proportion to ■ 

 the size of the blood-discs so is the size of the capillaries, and as the 

 capillaries may vary so will the size of the muscular fibres, the 

 nervous fibres, and in fact every tissue and organ of the body, so that 

 when the size of the blood-discs is known, some general idea may be 

 formed of the sizes of the different component parts of the other 

 tissues ; and should this mode of generalizing ultimately prove to be 

 applicable to the bone-cells as well, we shall be able not only to de- 

 termine the class of a fossil fragment, but to predict the size of the 

 blood-particles, and when they are once known the size and propor- 

 tions of the other soft tissues may at once be inferred. 



The laws of Nature are undeviating in the construction of the 

 skeleton of vertebrate animals : the same regularity in structure, the 

 same method of arrangement of the bone-cells, has existed from 

 the time when the surface of our planet was first inhabited by a. ver- 

 tebrate animal up to the present period. The largest bones of the 

 mighty Iguanodon (say of 100 feet in length), of the Ichthyosaurus — 

 the tyrant of the water in former ages, of the gigantic Tortoise of 

 the Himalaya range (some 20 feet in length), present no appre- 

 ciable difference, in their minute structure, from the pigmy race 

 of lizards that we now tread under our feet. The bones of the 



