I.— PHYSIOLOGY. 21.'5 



'aponeuroses that make the muscles fast to the bones, and tlic ligaments 

 that bind the bones to one another. The material that they deposit is 

 composed of inextensible fibres that lie, in the case of tendons at any rate, 

 so exactly and exclusively in the line of the resultant of the tension set 

 up in the muscle to which they attach themselves, that it is difficult to 

 believe that the disturbance which starts them producing their character- 

 istic secretion is anything else than the pull exerted on them by the muscle- 

 fibres to which they are attached ; the recurring external disturbances 

 that produce reversible states of tension in the muscle, indirectly producing 

 in them an irreversible reaction, which consists in the discharge of material 

 that by its inextensibility can transmit the tension along the line of the 

 force that provokes its deposition. In their simplest form cells of this 

 kind deposit the. wavy fibres in areolar tissue which, when straightened 

 out under the action of a displacing force, set a limit by their inextensibility 

 to the dislocation of the part first affected, and so distribute. the action of 

 the displacing force over surrounding areas. It is interesting to note that 

 the origin of cells of this kind has been traced to the mesothelium cells 

 that line tissue spaces and serous cavities, the clefts that make the gliding 

 displacements of parts over one another possible. The deposition of 

 fibrous material seems here, as in the tendons and ligaments, to be the 

 result of reaction to the recurring disturbances set up by displacements, 

 such, for instance, as those of the lungs, the alimentary tract, the heart 

 and pidsating vessels, and the deposition occurs in the line of strains set 

 up by the displacing forces. The service rendered by this behaviour of 

 the cells is that the fibres which they deposit, in virtue of their inextensi- 

 bility, limit the extent of displacement at any one point by distributing 

 it to surrounding parts. 



The other component of areolar tissue, the elastic fibres, is similarly 

 produced by other cells. These fibres take a straight course between their 

 attachments; displacements in the line of their deposition are rendered 

 possible by their stretching, and are recovered from by their elasticity. 



The contribution made by such cells to the fabric of the body appears 

 to result from the recurring operation of disturbances, to which they 

 react by depositing fibres along the lines of disturbance. 



More striking are the properties of cells upon which the formation of 

 the skeleton depends. The cells that make bone not only secrete fibrous 

 collagen, they also encrust the fibres with insoluble lime-salts, and it has 

 long been subject of comment that the rigid bone that results always 

 comes to lie in the line of prevailing strains and stresses. The analysis of 

 the structure, for instance, of the head and neck of the human femiir, 

 by Wolff and others who have followed him, shows how strictly this is 

 true. Calculations prove that no particle of bone lies anywhere but where 

 the strains dictate. We can predict with certainty, it seems, that it will 

 be found that bone-cells are composed of material that in reacting to 

 physical forces directs, in constant relation to the line of action of those 

 forces, the de})osition of the substances which make up this connective 

 tissue. Bone can only arise where strains and stresses set up this reaction, 

 and the greater the strain or stress the denser the deposit. When a bone 

 is fractured many bone-cells are dislodged, and, in the abundance of nutri- 

 ment that ruptured vessels supply, these cells, released from their 



