68 



HUMAN PHYSIOLOGY 



The disposition of the fibers in a mass of muscle may 

 be curiously varied. They may be laid parallel through- 

 out or they may have quite another grouping. When a 

 muscle has the character known as penniform we can 

 recognize the arrangement as possessing the greatest 

 value. An example of a penniform muscle is the 

 gastrocnemius, the chief calf muscle, 

 as it may be found in the frog. 

 A longitudinal section shows that 

 there is an internal core of con- 

 nective tissue which is really an 

 extension of the upper tendon by 

 which connection is made with the 

 thigh-bone. From this core the 

 fibers run outward and downward. 

 They are short and seem to be in- 

 serted in the superficial envelope 

 of the muscle. 



When the gastrocnemius con- 

 tracts, the external layer is drawn 

 up and the lower tendon (tendon 

 of Achilles) is lifted with it. The 

 central connective tissue is motion- 

 less. The core furnishes a place of 

 origin for a vastly greater number 

 of fibers than could be accomo- 



pull in the second case 

 is that of a far greater 

 number of fibers. 



FIG. 13. A band of 

 parallel fibers compared 

 with a penniform arrange- 

 ment. The former can 

 lift through a longer in- 

 terval, the latter with 



much more force. The dated, if they ran parallel from the 

 knee to the heel. The power of the 

 muscle is correspondingly increased. 

 At the same time its range of move- 

 ment as measured at the lower end is much reduced. In 

 effect, a relatively long bundle of muscular tissue, adapted 

 by its shape to have a place in the leg, is given the prop- 

 erties of a short muscle of great thickness. (Fig. 13.) 



Muscle Contraction. Muscles of the order which we 

 are now considering are not automatic. On the contrary 

 they are thrown into contraction because they are con- 

 nected with the central nervous system which presides 



