440 



THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



Fick and his pupils have made absolute determinations of the amount 

 of heat developed in muscular activity, and the amount of work done at the 

 same time. Some of their results are brought together in the following table. 

 In these experiments the load was allowed to fall again after each contraction, 

 so that the observed quantity of heat expresses the total output of energy. 

 Each experiment consisted of three maximal contractions following one 

 another in rapid succession. 



We see that under a maximal stimulus and with increasing load the ratio 

 of work: heat changes in favor of the former. With the least load the total 

 production of energy is 16.7 times the amount of work done, whereas with 

 the heaviest load it is only 3.7 times as much. In other experiments a still 

 greater part of the total production of energy appeared as mechanical work. 

 But as a rule in the frog's muscle cut out of the body by far the greatest 

 part of the energy developed in contraction is used for the production of heat. 



5. THE CENTRAL INNERVATION OF A SKELETAL MUSCLE 



Each one of the muscles of the extremities receives motor- nerve fibers 

 from several successive nerve roots. This is most clearly seen in the case of 

 the sterno-cleido-mastoid and the trapezius of man which are innervated by 

 both the spinal accessory and the cervical nerves. This fact alone appears 

 to indicate that under normal circumstances some of the fibers of a muscle 

 are not thrown into action, but that the muscle has the power of contracting 

 partially. The following experiment by Gad confirms this conclusion. 



The lumbar plexus of the frog conveys nerve fibers to the gastrocnemius by 

 two roots. If with a light load the muscle be given a tetanic stimulus directly 

 or indirectly by one or by both of these roots the contractions are of equal size. 

 But if the tension developed in the muscle in tetanus be studied by means of 

 the apparatus figured on page 414, it is found to be less when the stimulus is 

 applied to only one root than when applied to both or to the muscle directly, 

 and that in the latter two cases the tension is equal to the sum of the tensions 

 developed by separate stimulation of the two roots. The result goes to show 

 that on stimulation of different nerve roots, not the whole muscle but only cer- 

 tain of its fibers are excited, or in other words that each nerve root produces a 



