On the Agency of Nerves. 273 



less of faintness, with a weak and often slow pulse. All must 

 admit that this last change on the action of the^eart is the 

 consequence of an alteration (viz., a diminution,) of its con- 

 tractile power ; and is it not almost self-evident, that the former 

 change must be the consequence of an increase of that power ? 

 It is certain, that the heart of a person under the influence of 

 exciting passions, contracts more frequently and strongly on the 

 blood which it receives, than usual. The only measure that we 

 have, or can have, of the contractile power of the heart, is the 

 frequency and strength of these contractions. It is allowed, I 

 believe, on ail hands, that this change of its action is produced 

 through the medium of the nervous system ; and this being so, 

 is it a very new or untenable opinion,or rather is it not a tole- 

 rably correct expression for this acknowledged fact, to say, that 

 the heart is liable to increase, as well as to diminution, of that 

 vital power, by which it contracts on the blood, in consequence 

 of impressions made on the nervous system* ? 



I should not have dweilt so long on this part of the subject 

 if it had not appeared to me of considerable importance, in 

 various departments, both of physiology and pathology, to keep 

 in view the difference between the cases where muscular fibres 

 are directly stimulated to contraction by excitation of their 

 nerves, or of the brain, and those where causes acting on the 



• Dr. Philip speaks of the difficulty of understanding, how a power, not 

 derived from the nervous system, can be liable to increase from what he 

 calls nervous influence. This difficulty seems to me to lie more in words 

 than reality. If the contractile power were a tangible or appreciable 

 substance^ as some physiologists have supposed, we should be much at a loss 

 to understand, how any change taking place in the nerves should increase 

 what the nerves did not originally bestow ; but when it is distinctly un- 

 derstood, that by the term irritability or contractile power, we mean merely 

 to express the fact, that those parts which are possessed of this property 

 are thrown into contraction under certain circumstances, and do not at- 

 tempt any explanation of this fact, it appears to me not at all more difficult 

 to understand, that this property should be liable to increase, from causes 

 acting on the nervous system, than that it should be liable to diminution, 

 as Dr. Pliilip allows that it is ; or than that it should be liable to increase 

 from elevation, and to diminution from reduction of temperature ; as we 

 know that it4s, although no one supposes that irritability is derived from heat. 



