190 LOCOMOTION. [CHAP. vn. 



and this in very different degrees, according to varieties in the same 

 causes which affect the speed of its coagulation. No one will pre- 

 tend that this is not as much a property of living fibrine when solid, 

 as that of coagulating is of the same substance when fluid; and the 

 correspondence between the coagulated living fibrine of the blood 

 and the living sarcous tissues in chemical constitution, in the posses- 

 sion of contractility, and in the modes in which that vital property 

 in both is affected by similar causes, adds strong confirmation to the 

 opinion we have expressed, that contractility is a property of the 

 living muscular substance as such. 



But contractility does not vary in its durability alone ; it also 

 presents great differences in regard to its aptness to excitation by 

 stimuli: and it would appear that these characters are always 

 cater is paribus, in an inverse relation to one another. In birds and 

 insects, which have for the most part to sustain themselves by very 

 energetic and rapid muscular movements in the air, the excitability 

 is extreme ; and certainly the motions performed by these creatures 

 far exceed in precision, regularity, and frequency those of any other 

 animals. 



The rigor mortis, or stiffening of the body after death, is due to a 

 contraction of the muscles. If the contractility of a muscle be en- 

 during, the rigor comes on late and lasts long; but if it be evanescent 

 and its character excitable, the rigor begins very soon and quickly 

 terminates. This is true in different individuals and classes of ani- 

 mals, and corresponds entirely with what we have already said of the 

 varieties of this property. Its cause is obscure, and may be com- 

 plex; but its resemblance to the contraction of fibrine after recent 

 coagulation is too obvious to be overlooked. Its nature is shewn 

 by the preceding observations (p. 180). 



We have the power, at will, under certain limitations, of pro- 

 ducing, checking, and regulating the amount of contraction in tl 

 voluntary muscles ; and, as a necessary part of this power, we 

 able to appreciate, by certain sensations originating in the muscle 

 what precise degree of contraction is present in each. This Iat1 

 is only that modification of common sensibility which belongs 

 muscle. It has been termed the muscular sense. In it we 

 a most important aid to the sense of touch, being able accurately 

 vary the position and amount of pressure on external objects 

 voluntary accordance with the impressions these communicate 

 the sensorium through the tactile nerves ; and by it we are able 

 estimate with nicety the amount of muscular power required 

 balance various resistances, as weights, &c. In general, th( 



