337 



cretion ; which may, likewise, be obtained in a separate state, calcining 

 the osseous part. The different proportions of the saline to the organized 

 part, vary considerably at different periods of life : the bones of the em- 

 bryo are, at first, quite gelatinous. At the period of birth, and during the 

 first years of life, the organic part of the bone is in greater proportion ; 

 the bones are less apt to break, more flexible, possessed of more vitality, 

 and, when fractured, are more speedily and more easily consolidated, In 

 youth, the two constituent parts are nearly in equal quantities; in adults, 

 the calcareous earth* alone forms two thirds of the osseous substance. 

 At last, gradually increasing in quantity, it displaces, in old people, the 

 part that is organized : hence their bones are weaker, more liable to frac- 

 ture, and unite less readily. One may therefore say, that the quantity of 

 phosphate of lime deposited in the bones, is in the direct ratio of the age; 

 and that, on the contrary, the energy of the vital faculties of these or- 

 gans, their flexibility, their electricity, their aptitude to become conso- 

 lidated, when their continuity is destroyed by accidents, are in an inverse 

 ratio. 



Anatomists distinguish in bones three substances, which they term 

 compact, spongy, and reticular. The first, which is the hardest, collect- 

 ed in the centre of the long bones, where the greatest stress of the efforts 

 applied to their extremities rests, gives to the bone the strength which it 

 required. Its formation has been explained, in various ways; some have 

 maintained that it owed its hardness to the pressure applied to its middle 

 part by the tv/o extremities of the bone ; in the same manner as the stalk 

 and the roots press against the collet^ of a plant. Haller thinks it is caused 

 by the pulsations of the nutritious arteries which penetrate into the long 

 bones, at their middle part; why then is their structure different at their 

 extremities where they receive arteries equally large and more numerous? 

 In the process of ossification, this substance appears first in the centre of 

 the longbonesj; and this confirms the assertion of Kerkringius, whosays, 

 that our long bones begin to ossify, in those points where they have to 

 resist the greatest pressure. 



The spongy substance is found within the short bones, and at the ex- 

 tremities of the long ones, where'its accummulation is attended with two 

 advantages, that of giving to the bone, without increasing its weight, a 

 considerable size, by which it may be articulated with the neighbouring 

 bones, by wide surfaces, so as to give firmness to their connexions; this 

 confirmation is attended with another advantage, that of avoiding the pa- 

 rallelism of the tendons which pass over the joints, in order to enlarge 

 the angle of their insertion in the bones, and to give more efficacy to mus- 

 cular action. The mechanical hypotheses proposed by Haller and Du- 

 hamel, to explain the formation of this spongy substance, are very unsatis- 

 factory, especially if it be considered, that in the gelatinous bones of the 

 embryo, the place that is to be occupied by the spongy substance, viz. 

 the extremities of the long bones, of which the rudiments begin to ap- 



* By chemical analysis of the bones, there have been discovered several other saline 

 substances mixed with the phosphate of lime ; but as this salt alone constitutes the 

 greatest part of the substance which gives to the bones their hardness, it has been par- 

 ticularly adduced. For an account of the chemical constitution of the osseous, texture, 

 see the Chapter at the end ot the APPENDIX Copland. 



f The part where the stem joins the root. Trans. 



* See APPENDIX, Note H H 



2 U 



