176 LOCOMOTION. [CHAP. vii. 



a contraction excited in the latter mode would be diffused over the 

 parts to which the nervous twigs irritated were supplied, and would 

 therefore frequently occur in parts at some distance. 



Having premised these words respecting the stimuli of muscle, 

 we proceed to consider what is known of the phenomena which 

 attend the act of contraction. It is evident that the subject we are 

 now approaching is one of primary importance ; because, on the 

 positive information regarding it, must chiefly depend our means of 

 udging of the conflicting hypotheses of the nature and laws of 

 action of the contractile force. 



A muscle, when contracted, is firmer than before ; but this 

 rigidity is proportioned rather to the intensity of the contractile 

 force exerted, than to the amount of shortening occasioned by it. 

 The circumstance, however, has led to the belief, that the act is 

 accompanied with a compression of the substance of the muscle into 

 a smaller compass ; but it is, on the contrary, well ascertained that 

 it gains in thickness what it has lost in length. The experiments 

 by which this fact is attested have been often repeated, and their 

 general results accord well together. If a muscular mass be made 

 to contract by means of galvanism in a vessel of water furnished 

 with a very delicate tube, the slightest diminution of bulk would be 

 at once indicated by the fall of the water within the tube ; but the 

 water, under these circumstances, is found to retain its level. Mr. 

 Mayo varied this experiment by selecting the heart of a dog,* 

 which, continuing to beat during some time, was in this way dis- 

 tinctly seen to undergo no change of size. 



The familiar practice of accelerating the flow from the vein at 

 the elbow, by desiring the patient to contract the muscles of the 

 fore-arm, does not, as is sometimes imagined, shew any diminution 

 of their bulk, but only a forcible increase of their lateral dimensions, 

 by which the deep veins are compressed within the inelastic sheath 

 of fascia, and the blood diverted into the superficial channels. In 

 those muscles which have a bulging centre, or belly, as the biceps of 

 the arm, the fibres are arranged in a curved form, and during con- 

 traction must tend towards a straight line in the direction of the 

 axis of the muscle. In such instances, the blood-vessels that tra- 

 verse their interstices must be in some degree compressed. 



If we examine under the microscope the contracted state of a 



morsel of the sarcous tissue, we find it to present essentially the 



same characters as that of the entire organ, a shortening in length, 



with a corresponding increase in thickness ; and this is true, how- 



* Anat. and Phys. Comment., vol. i. p. 12. 



: 



