CHAP. I.] VITAL PROPERTIES. 55 



direct exercise of vital force, the tissues are not the less dependent 

 on healthy vital action for the preservation of their peculiar pro- 

 perties in a state of integrity. Whoever will compare the compact 

 figure of a vigorous healthy man, accustomed to field-sports and 

 active exercises, with the relaxed, feeble, half-dislocated limbs of 

 an ill-nourished, hysterical woman, will readily perceive how great 

 an influence healthy nutrition must exert in preserving and improv- 

 ing the physical properties of the tissues. 



The vital properties manifest themselves by a change which 

 occurs in the molecules of certain tissues, as the result of a 

 stimulus applied. The change, thus produced, may be evident 

 from a visible alteration in the tissues stimulated ; or it may shew 

 itself through a secondary influence exerted upon some other 

 texture or organ, with which the stimulated tissue may be in 

 connexion. 



These properties exist in two tissues, namely, in muscle and in 

 nerve. A muscle, when stimulated, shortens itself; and, therefore, 

 it is said to possess the property of contractility. This power of 

 contracting, in obedience to a stimulus, is characteristic of muscle, 

 and probably occurs in no other kind of animal texture. The 

 stimulus may be direct irritation by mechanical means, or by gal- 

 vanism, or by some chemical substance; but the natural one, 

 during life, is propagated by the nerves. 



In nerve, the vital changes are unaccompanied by any altera- 

 tion in the tissue itself, which is appreciable by our senses. The 

 excitation or irritation of the nerve may be manifested in three 

 ways : first, by its inducing the contraction of the muscle which 

 it supplies ; secondly, by its exciting contraction, in muscles which 

 it does not supply, through a change wrought in the nervous 

 centre; thirdly, by its exciting a sensation. The same stimuli, 

 which we have mentioned as capable of exciting muscular con- 

 traction, will produce these effects in nerves; and the will, and 

 other emotions of the mind, are capable of stimulating nerves 

 which are connected with the brain, and exciting action in the 

 muscles to which they are distributed. 



That a nerve, when irritated, may excite a sensation, it is 

 necessary that it shall be in connexion with the brain. The bodily 

 feelings of pain or pleasure are thus produced, through the medium 

 of what are called sensitive nerves, or nerves of common sensation ; 

 and we say that the sensibility of any tissue is great or small, 

 according as it is supplied with such nerves in more or less quantity. 

 Tendon, in which probably few nerves exist, is a tissue of low 



