64 MATERIAL COMPOSITION OF MAN. 



general. They are found to vary in consistence, some being hard, 

 and others soft ; as well as in colour, transparency, &c. They have, 

 also, physical properties, analogous, indeed, to what are met with in 

 certain inorganic substances, but generally superior in degree. These 

 are flexibility, extensibility, and elasticity, which are variously com- 

 bined and modified in the different forms of animal matter, but exist to 

 a greater or less extent in every tissue. Elasticity is only exerted 

 under particular circumstances: when the part, for example, is put 

 upon the stretch or compressed, the force of elasticity restores it to its 

 primitive state, as soon as the distending or compressing cause is with- 

 drawn. The tissues, in which elasticity is inherent, are so disposed 

 through the body, as to be kept in a state of distension by the mechani- 

 cal circumstances of situation; but, as soon as these circumstances are 

 modified, elasticity comes into play, and produces shrinking of the sub- 

 stance. It is easy to see, that these circumstances, owing to the con- 

 stant alteration in the relative situation of parts, must be ever varying. 

 Elasticity is, therefore, constantly called into operation, and in many 

 cases acts upon the tissues as a new power. The cartilages of the ribs, 

 joints, &c., are in this manner valuable agents in particular functions. 



We have other examples of the mode in which elasticity exhibits 

 itself, when the contents of hollow parts are withdrawn, and whenever 

 muscles are divided transversely. The gaping wound, produced by a 

 cut across a shoulder of mutton, is familiar to all. Previous to the 

 division, the force of elasticity is kept neutralized by the mechanical 

 circumstances of situation, or by the continuity of the parts; but as 

 soon as this continuity is disturbed, in other words, as soon as the me- 

 chanical circumstances are altered, the force of elasticity is exerted, 

 and produces recession of the edges. This property has been described 

 under various names, tone or tonicity, contractilite de tissu, contractilite 

 par defaut d 1 extension, &c. 



The other properties, flexibility and extensibility, vary greatly ac- 

 cording to the structure of parts. The tendons, which are composed 

 of areolar tissue, exhibit very little extensibility ; and this for wise 

 purposes. They are the conductors of force developed by muscle, and 

 were they to yield, it would be at the expense of the muscular efforts; 

 but they possess great flexibility. The articular ligaments are very 

 flexible, and somewhat more extensible. On the other hand, the fibrous 

 or ligamentous structures, which are employed to support weights, or 

 are antagonists to muscular action, as the ligamentum nuchse, which 

 passes from the spine to the head of the quadruped, are very extensible 

 and elastic. 



Another physical property, possessed by animal substances, is a kind 

 of contractility, accompanied with sudden corrugation and curling. 

 This effect, which Bichat terms racornissement, is produced by heat, 

 and by chemical agents, especially the strong mineral acids. The 

 property is exhibited by leather when thrown into the fire. 



An effect, in some measure resembling this, is caused by the evapo- 

 ration of the water that is united to animal substances. This consti- 

 tutes what has been called the Jiygrometric property of animal mem- 



