66 



HUMAN PHYSIOLOGY 



necessary to emphasize the teaching that even here 

 connective tissue is present. It exists in a sheath, 

 enveloping the muscle, and it also subdivides it in- 

 ternally, blocking off its substance into many bundles in 

 the way which is so evident in a beefsteak. It is easy 

 to forget that connective tissue is 

 everywhere associated with the con- 

 tractile; it is in fact indispensable to 

 the mechanism. It may be said to 

 constitute a harness by means of which 

 the innumerable living units trans- 

 mit their combined force. 



It will be found upon examination 

 that the moving bones are levers with 

 such relations that the pull of the 

 muscles is generally upon the short 

 arm. It is, in other words, nearer 

 the fulcrum than is the load to be 

 lifted. The organization is therefore 

 one which secures speed and extent 

 in motion at the expense of sheer 

 force. When the forearm is held 

 _, , . horizontal and a 10-pound weight is 



FIG. 11. The bi- ,.,111,1 



ceps is shown with its upon the palm of the hand, the actual 

 chief antagonist, the tension maintained by the biceps and 



bends S the T e h ibow C a e nd one or two auxiliary muscles must be 

 is termed a flexor more like 100 pounds. The student 

 muscle, the triceps, may find himself reluctant to believe 

 oountr g m o a ve ment, h i e s this, but a little attention to the pro- 

 an extensor. portions of the parts concerned will 



compel him to assent to the conclu- 

 sion. The muscles which bring the lower jaw against 

 the upper are so attached as to apply almost their whole 

 power in the region of the molars and a pressure of 270 

 pounds has been recorded between these teeth. 



Muscle Fibers. Let us now attend to the nature of the 

 living units just mentioned. They are the fibers of the 

 muscle. A single one has about the dimensions of a 



