motory vibrations is the residue of all the vibrations excited in the various parts of the body; and the third arises chiefly from the heat of the blood. We may expect therefore, that the heart should contract either more frequently, or more strongly, or both, when the body is heated. And thus it is, as may appear from observations upon the pulse in fevers, in sleep, after eating, in pains or distempers attended with an increase of heat, &c. in which the motion of the blood is increased; whereas in nervous pains attended with coldness of the extremities, the pulse is low and slow.
Fourthly, In the declension of fevers the pulse is quick and feeble. It is feeble, because the whole body is so; and quick, partly from the new habit superinduced by the heat in the beginning of the fever; partly, because in fevers the heart is always kept nearly full, i.e. nearly at such a degree of distention as incites it to contraction, the principal causes of which are the weakness and inactivity of the body: hence in general the pulse is quick and feeble in persons of relaxed habits; the contrary in strong ones.
Fifthly, The pulse is quick in young and small animals; slow in old and large ones. For this, various reasons may be assigned; as first, that if the velocity wherewith the sides of the heart move towards each other be the same, the contraction must be sooner accomplished in small hearts than in large ones. Secondly, that the fibres in young animals are irritable, and soon excited to contraction, by distention, &c. Thirdly, that the contraction is performed slowly in old animals; and fourthly, that short fibres are perhaps sooner excited to contraction than long ones, it being necessary perhaps, that the vibrations should be reverberated from each end of the fibres, for many successions, before they can rise to a certain pitch. It agrees with this, that reciprocal motions are more frequent in general, and cæteris manentibus, as the animal is less; that the limbs have both long and short flexors and extensors, the first for great degrees of motion, the last for making a quick beginning; that the capsular ligaments of the joints have short muscular fibres inserted into them, in order to keep them from being pinched between the bones in the motions of the joints, as Winslow has observed; which they could not do, had not their contraction the start of the contractions which move the joint; and lastly, that the fibres which compose the heart, are all of equal lengths, according to Dr. Stewart’s analysis of them. See Phil. Trans. No. 460.
Sixthly, The heart may move incessantly without fatigue, if we only suppose the recruits to be sufficient, and the degree of motion to be within due limits. And it may be, that in labouring men the muscles of the limbs are as much exerted upon the whole as the heart. The warmth in which the heart is kept, and its receiving nerves from the eighth pair and intercostal, which seem to be particularly exempted from venal compression,
