PHYSICAL PROPEKTIES OF TISSUES. 63 



In youth the tissues have less cohesion than in adult life, from the 

 greater preponderance in the former period of water ; while the cohesion 

 again declines in old age, especially in bone and muscle, even though 

 the proportion of water present also diminishes, from changes in the 

 quantity of inorganic elements. 



The cohesion of airy tissue is not uniform in all directions, but, as is 

 well known, certain tissues may be ruptured in one direction more 

 readily than in another ; thus, a costal cartilage is more readily broken 

 transversely than longitudinally. This is even more marked in fibrous 

 tissues, such as a tendon, where it is much easier to separate the longi- 

 tudinal fibres than it is to rupture them by traction. This may be 

 explained by the fact that the cohesion of any tissue is the resultant of 

 the forces which holds the ultimate molecule of the tissues together, as in 

 a single fibre of connective tissue, and of the adhesive foi-ce, which 

 through the mediation, ordinarily of cement substance, holds several 

 collections of similar molecules together. 



The forces which maj' act on a tissue to destroy its cohesion may 

 operate in four different ways : by traction, by pressure, by flexion, and 

 by torsion. All the different tissues behave differently to each of these 

 modes of action. 



The resistance to traction is measured by the force required to tear 

 apart the molecules of any tissue ; hence, the force required to produce 

 tearing in any tissue must increase with the cross-section of the tissue 

 subjected to strain, and when the cohesion of two different tissues is 

 compared in this respect the comparison must always be reduced to a 

 unit of cross-section. Thus, in the following table the numbers represent 

 the breaking weight in kilogrammes for every square millimeter of 

 surface (Wertheim): — 



Bones, . . • 7 76 



Tendons, 6.94 



Muscles, 0.054 



Nerves, 0.93 



Arteries, °- w 



Veins, . ° 13 



This resistance to traction is of great importance in the mechanics 

 of the organism. The cohesion of the bones, tendons, ligaments, and 

 muscles permits of the accomplishment of mechanical work, while the 

 resistance to distension of the different membranes of the body, such as 

 the aponeuroses, fibrous membranes, etc., is of great value in numerous 

 physiological operations. 



The resistance to pressure is especially seen in the bony skeleton, 

 articular cartilage, and intervertebral disks. In the bones this is especially 

 very marked. Thus, it has been found that from 1110 to 2300 kilo were 

 required to crush a cube of bone from the compact substance of the 



