64 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



bones of the extremity 5 millimeters thick, while only 100 kilo were 

 required to crush a cube of the same size from the spongy substance. 

 The cohesion of the compact substance measured in this way decreases 

 to about the same degree when either the organic matter or the lime salts 

 are removed; it also decreases greatly when the water is removed, 

 showing a deviation from the general statement above made. This resist- 

 ance to pressure plays an important role in the support of the body in 

 standing, walking, and jumping, and in the protection from injury of such 

 important organs as the brain, spinal cord, lungs, and heart. 



The resistance to pressure in the osseous system decreases with age. 

 Thus, Pick has found that a prism of bone 1 square millimeter in size 

 from a man 30 years of age was crushed by a weight of 15.03 kilo, while 

 a similar piece from a man aged 74 years would not sustain a weight of 

 4.33 kilo. The bones of different animals also show great differences in 

 their resistance to pressure. 



The resistance to flexion and tursion possessed by the different tissues 

 of the body also comes into play in certain physiological operations. 

 Thus, in inspiration the ribs and costal cartilages undergo a slight 

 amount of twisting and bending through the action of the inspiratory 

 muscles, and regain their position during expiration. So, when a weight 

 is lifted and held horizontal by the hand the resistance to flexion pre- 

 vents bending of the bones of the arm. 



The cohesion of the tissues is always greatest in the direction in 

 which the forces which act on those tissues is usually exerted. Thus, 

 when tissues are ordinarily subjected to the force of ti'action, their co- 

 hesive force is most developed in a longitudinal direction, and such tis- 

 sues, like tendons and ligaments, are fibrous in structure. When the 

 pressure or force to which tissues may normally be subjected does not 

 lie in any one but in many different directions, as in the resistance 

 which serous membranes and aponeuroses offer to distension, such mem- 

 branes are also fibrous in structure ; but the fibres, instead of being 

 parallel to each other as in tendons, in which traction is the only force 

 to which they are subjected, are interlacing and cross each other in 

 every direction. Finally, when pressure is the force which must be 

 resisted, we find the tissues taking the form in which such resistance 

 may be best offered ; the compact bony tissues are therefore arranged in 

 arches, as in the head of the femur, or in the form of hollow tubes, 

 as in the shafts of the long bones, — two forms which, with the greatest 

 economy of material, offer the greatest resistance to pressure. In the 

 case of the femur its upper end is not only subjected to pressure' from 

 the weight of the body, but also to flexion ; for the head of the femur 

 is not in a line with the long axis of the bone, but lies to one side and 

 is, connected with the shaft of the bone by an oblique neck. The 



