x] of the Nervous System. 291 



"ment and have no need of any abundant afflux of spirits, 

 " either animal or vital, by which they are inflated, and being 

 " so shortened carry out the movements ordered by the brain." 



We nowadays avoid the concomitant changes in the volume 

 of blood present in the arm, and take not a whole limb of 

 man but the bloodless muscles of a frog; but otherwise the 

 plethysmographic proof we now use is identical with that of 

 Glisson. 



But Glisson's irritability and his notable experiment were 

 like Mayow's igneo-aereal spirit forgotten as the seventeenth 

 century passed into the eighteenth. We have to wait until the 

 middle of the latter century, when the truth was brought to 

 light again by the sagacious Haller in his views of nervous 

 action and its relation to muscular contraction. To these we 

 must now turn. 



In his Elemeiita, Haller in treating of the subject begins 

 by discussing the contractile force in general. " There is 

 " widely present not only in the animal but also in the 

 " vegetable kingdom a contractile force by which the elements 

 u of fibres are brought nearer to each other. This not only 

 "seems to be the cause of cohesion in general, but is 

 " rendered manifest by the fact that a fibre drawn out 

 " Fengthways when let go very soon returns to its previous 

 "length, and never lays aside the effort to become shorter 

 " until it has so returned to its previous length." This 

 is more properly the elastic force. Besides this there is a 

 contractile force by which the tissues dead or alive shrink 

 when treated in various ways, when for instance they are 

 heated. A contractile force of such a kind is present in almost 

 all animal tissues, unless it be the very soft and pulpy ones like 

 brain or the very hard ones like bone and teeth. But there is 

 in addition a special contractile force proper to muscles alone. 

 "In a living animal, or in one only just dead there very 

 "frequently appears spontaneously in muscular tissue a swift 

 " vivid contractile movement by which the ends of the muscle 

 "are alternately brought nearer to the middle belly and then 

 "again recede from it." And even when this contractile 

 movement does not spontaneously appear, it may be excited if 



19—2 



