664 



CIRCULATION. 



arteries of the extremities is a little later than 

 the beat of the heart on the ribs and the pulse 

 in the arteries in the immediate neighbourhood 

 of the heart. This retardation has of late been 

 more distinctly pointed out by Dr. M'Donnell 

 of Belfast,* and by Weber of Leipsig.f It is 

 much more marked in some persons than in 

 others, and is always most perceptible when 

 the circulation is slowest. \Vith a little atten- 

 tion we can thus observe a distinct succession 

 in the occurrence of the beat of the apex of the 

 heart at the ribs, the pulse in the carotid, 

 facial, radial, and posterior tibial arteries, the 

 interval between each of which, though very 

 small, being yet appreciable by the finger. 

 Weber states that the retardation of the pulse 

 in the foot after that of the beat of the heart 

 amounts to not more than one-seventh part of 

 a second. We have ourselves confirmed by 

 experiments on several individuals the most of 

 these facts relating to the later pulse in the 

 more remote arteries. The cause of the retar- 

 dation is obviously the elasticity and yielding 

 of the arterial panetes ; for were the arteries 

 rigid tubes, it is manifest that the impulse of 

 the heart would be felt at one and the same in- 

 stant of time throughout the whole of the 

 branches ; but as these vessels yield to disten- 

 sion, that part of them to which the distending 

 force is immediately applied is first dilated, 

 and this dilatation does not reach immediately 

 the remote parts. 



The pulse has been correctly compared to 

 the propagation of an undulation or wave on 

 the surface of water; for the successive im- 

 pulses of the heart are first given to the column 

 of blood in the commencement of the aorta; 

 this column communicates these impulses to 

 the arterial parietes and tends to distend them. 

 The parietes re-act against this distending force 

 arid compress the adjoining part of the column 

 of blood, from which the impulse passes to the 

 next part of the aorta; and so the pulse, gradu- 

 ally passing on from the trunks to the smaller 

 bmnches, becomes less and less perceptible as 

 the force of the heart is equalized by the elastic 

 resistance of the coats of these vessels.^ 



The pulse is still perceptible in very small 

 arteries: Haller states that he was unable to 

 perceive any in small arteries of one-sixth of a 

 line in diameter, an observation which does 

 not, however, prove the flow of the blood to be 

 uniform or without jerks even in vessels of 

 this size, for Spallanzani || observed pulsations 

 in arteries of this small size; and the microsco- 

 pic observation of the circulation in transparent 

 parts by Haller himself, Spallanzani, and 

 others, shews that the visible impulse of the 



* At the Meeting of the British Sclent. Associat. 

 in Dublin. 



t De pulsu in omnib. arter. plane non synchro- 

 nico. Annot. Academ. Leipzig, 1834. 



^ Young's Croonian Lecture on the Functions of 

 the Heart and Arteries, in, his Introduction to Me- 

 dical Literature. 



Mem. surle Mouvement du Sang. Laus. 1756. 

 Translated. 



|| Exper. sur la Circulation, in French, by Tour- 

 des. Paris, An 8. In English, by Hall. Lond. 

 1801. 



heart is communicated to the blood in the 

 smallest of those vessels, which have distinctly 

 the characters of arteries. 



The pulse being nothing else than the beats 

 of the heart transmitted through the arteries, 

 the consideration of the variations in force or 

 frequency to which it is subject belongs more 

 properly to the subject of the functions of the 

 heart. In this place we shall only mention 

 the mean of the usual number of pulsations of 

 the arteries in the space of a minute as they 

 occur at different periods of life. 



Child before birth 140150 



Newly-born infant 1 30 140 



Child one year old ........ 120 



Two years . . . . 108 



Three years 95 



Seven years 85 



Age of puberty 80 



Manhood 75 



Old age 6050 



d. Vital properties of the arteries. In the 

 view we have hitherto taken of the arterial circu- 

 lation we have considered the coats of the arte- 

 ries as endowed with physical powers only, and 

 we have alluded to no other phenomena of the 

 motion of the blood than those which appear 

 to be connected with their elasticity. We have 

 now to direct our attention to the more strictly 

 vital and contractile powers of the arteries, 

 which constitute them an independent source 

 of force, and to examine how far the operation 

 of such powers may modify the flow of the 

 blood. We shall here discuss more in detail 

 the questions whether the heart is to be regard- 

 ed as the only source of the power by which 

 the blood is impelled, and the bloodvessels 

 merely as the modifiers or regulators of the 

 force generated by the heart's contraction or 

 whether the arteries do not, by their own inde- 

 pendent power, contribute to the propulsion of 

 the blood. 



Physiologists are very much divided in their 

 opinions upon these questions, some regarding 

 the heart as the sole moving power, some suppo- 

 sing the bloodvessels to be the principal, the 

 heart a subordinate cause of motion; and others 

 adopting various modifications of these oppo- 

 site views. Many who agree in considering 

 the heart's action as insufficient to propel the 

 blood through the smaller bloodvessels into the 

 veins, differ as to the cause of the additional 

 power supposed necessary for the maintenance 

 of the circulation ; the larger and middle sized 

 arteries being looked upon by some as highly 

 contractile, and in consequence of this, the 

 agents of propulsion ; the capillaries being re- 

 garded by others as the most efficient promoters 

 of the flow of the blood within the bloodves- 

 sels. We must, for the present, confine our 

 remarks to the first of these, or the opinion that 

 the larger arteries are mainly or in part the 

 agents of the propulsion of the blood. 



That the arteries have the power of changing, 

 to a certain extent, the quantity of blood which 

 passes through them, and of thus modifying the 

 circulation by their own independent powers, 

 there can be no doubt, from the occurrence of 

 unequal distributions of blood, or of local de- 



