OF THE MOTOEY APPARATUS IN MAN. 135 



by means of these instruments, a movement slower or more 

 rapid. 



Thus let us suppose that the power (p) acts on the lever 

 (a r) so as to cause it to pass to the point of insertion c, a 

 space of 5 in a second, it will displace at the same time the 

 extremity (r) of the lever, and will cause it to arrive at b with 

 a quickness equal to 25, 



for the distance passed v ^ r m j 



through in equal times hy 



this point will be five times 



greater than that passed 



through by the point d. 



With a force equal in 



rapidity to 5 we produce, 



by applying it to the point 



c, the same result as if we 



applied to the point r a 



force having a quickness 



equal to 25. But all that 



is gained in rapidity is lost in power or force ; for it is chiefly 



by extending the arm of the lever of resistance to a length 



disproportionate to that of the force that we obtain these 



results. 



Now, in the animal economy almost all the levers are so 

 disposed as to favour rapidity of motion at the expense of 

 force. Thus, in lowering the extended arm, if the rapidity 

 with which the muscles contract be such that their point of 

 insertion be displaced three inches in a second, the extremity of 

 the limb will pass from its original position with a rapidity 

 of nearly three feet per second. 



Description of the Motory Apparatus in Man. 



268. The motory apparatus as we have already 

 mentioned is composed in man and all the vertebrata of the 

 skeleton, the muscles, and the articular apparatus. 



The skeleton is divided into the head, trunk, and extremities. 



269. The Head. The skeleton of the head is composed 

 of two portions the cranium and face. The cranium lodg- 

 ing the brain, cerebellum, and pons, with their membranes, 

 the vessels proceeding to and from them, and the roots of the 

 cranial nerves, is composed of eight bones ; the frontal or 

 coronal (Fig. 89 f) : the two parietal, p ; the two temporal, 



