378 MUSCULAR MOTION. 



which lubricates them; and facilitates the play of the tendons, for the 

 passage of which they are destined. 



The mechanical structure of bone is a laminated framework incrusted 

 by an earthy substance, and penetrated by exhalant and absorbent 

 vessels, arteries, veins and nerves. M. Herissant 1 appears to have been 

 one of the first who stated, that bone is essentially composed of two 

 substances: the one a cartilaginous basis or parenchyma, giving form 

 to the part; the other a peculiar earthy matter deposited on this basis, 

 and communicating to it hardness. These two constituents can be readily 

 demonstrated ; the first, by digesting the bone in dilute chlorohydric 

 acid, which dissolves the earthy part, without acting on the animal 

 matter; and the second, by burning the bone until all the animal mat- 

 ter is consumed, whilst the earthy is left untouched. 



If we' take a long bone and divide it longitudinally, we find, that it 

 is composed of three different substances, all of which may, however, 

 be regarded as the same osseous tissue in various degrees of condensa- 

 tion. These are, the hard or compact substance ; the spongy or areolar; 

 and the reticulated. The first is in the most condensed form; it exists 

 at the exterior of the bone, and constitutes almost the whole of the 

 shaft. The second is seen towards the extremities of a long bone, 

 and in almost the whole of the short bones. In it, the laminae are less 

 close, and have a cancellated appearance, the cellules bearing the 

 name of cancelli. The reticulated substance is a still looser formation ; 

 the laminae being situate at a considerable distance; and the space 

 between filled up with a series of membranous cells, which lodge the 

 marrow. The marginal figures represent a longitudinal and a trans- 

 verse section of the same bone, in which this arrangement is well 

 exhibited. 



We have seen the advantages of the expanded extremities of long 

 bones, as regards the insertion of muscles; but it is obvious, that if 

 these portions of the bone had consisted of the heavy compact tissue, 

 the increased weight would have destroyed the advantages, that would 

 otherwise have accrued; whilst, if the shaft of the bone, exposed, as it 

 is, to external violence, had consisted of the spongy tissue only, it would 

 not have offered the necessary resistance. It is, therefore, formed 

 almost entirely of the compact tissue; so that a section of one inch, taken 

 from the body of the bone, will not differ essentially in weight from an 

 inch taken from the extremity. Nor does the cavity within the bones 

 dimmish their strength, as might at first sight be presumed. By en- 

 larging the circumference, the contrary effect is produced ; for we shall 

 see, in the mechanical proem to the particular movements, that of two 

 hollow columns, formed of an equal quantity of matter and of the same 

 height, that, which has the larger cavity, is actually the stronger. A 

 very important use of the cancellated or spongy texture of the bones 

 was suggested by a distinguished individual of this country, to whom 

 surgical science, in particular, has been largely indebted. Dr. Physick 2 

 asserts, that it serves to diminish, and, in many cases, to prevent, con- 



1 Memoir, de 1'Academ. des Sciences de Paris, pour 1758, p. 322. 

 3 Homer, Special and General Anatomy, &c., 5th edit., Philad., 1843. 



