184; THE MOTION OF THE BLOOD. 



" And indeed, after all, it appears that the diastole of an 

 artery is owing to a lateral distension given by the impetus of the 

 blood, so that the coats are expanded, and, by their elasticity, 

 the next moment reacquire their natural thickness. To the same 

 impulse may be ascribed the lateral motion of the axis, observ- 

 able in the larger arteries, if serpentine and lying in loose cellular 

 substance. 



" The genuine systole, produced by a contraction of their sub- 

 stance, scarcely occurs, probably, while the heart acts with vigour, 

 but may, when they are unusually influenced by local- stimulants ; 

 whence the pulse during illness is very different in different arte- 

 ries of the same person at the same time ; or when the action of 

 the heart itself fails," &c. 



Most physiologists grant to the capillaries irritability, tonicity, 

 or organic contractility; but some deny that arteries possess 

 muscular properties. Bichat's objections are, the absence of con- 

 traction on the application of stimuli to them, the much greater 

 resistance of the middle coat to a distending force than of mus- 

 cular parts, and, lastly, the difference of the changes which it and 

 muscles undergo both spontaneously and by the action of other 

 substances. 11 Berzelius has multiplied the latter description of 

 proofs. x However this may be, I must remark, first, that the 

 capillaries have certainly vital powers of contraction as fully as 

 any parts of the body. This appears in their various degrees of 

 local dilatation and contraction, under inflammation, passions of 

 the mind, &c. When different stimuli are applied to them, they 

 are seen under the microscope locally to experience various de- 

 grees of contraction and dilatation, and this even after connection 

 with the heart has been cut off by absolute excision of this organ.y 

 Under similar circumstances, when no stimulus was applied, the 

 blood was seen by Dr. Hastings often to cease, indeed, to flow, 

 but still to oscillate. If the capillaries are allowed to possess 

 organic contractility, it is impossible to say in which point of the 

 arterial tract it begins. 



The evidence of muscular fibres is not necessary to irritability. 

 The iris and uterus are strongly endowed with irritability, but 

 their muscularity is disputed by many. No muscularity is dis- 



u Anatomie Generate, t. ii. x Traite de Chimie, t. vii. p. 84. sq. 



y See Dr. Wilson Philip, On Febrile Diseases: Dr. Thomson, Lectures on In- 

 flammation; Dr. Hastings, A Treatise on the Inflammation of the Mucous Mem- 

 brane of the Lungs. 1820. 



