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 140-150 Newly-born infant 130-140 Child one year old 120 Two years .. 108 Three years 95 Seven years 85 Age of puberty 80 Manhood 75 Old acre 60-50 d. Vitarproperties qf 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 vvhich 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 terminations of that fluid which take place in blushing, inflammation, and other states of the economy in which particular parts of the vas cular system become more or less filled with blood than usual ; for such variations in the distribution of the blood would be impossible, were an alteration in the powers of the heart alone the only means of modifying the circula tion. The questions, however, whether such
powers as are possessed by the arteries contri bute upon the whole to the progressiVe motion of the blood or modify only its distribution, are quite distinct from one another.
In its anatomical structure the fibrous coat of the arteries differs considerably from muscu lar substance, and appears to resemble more nearly the yellow elastic ligamentous tissue. Its fibres are less mixed with cellular substance than those of muscles ; they are also more dry, hard, and friable, less coloured, and, accord ing to Hodgkin and Lister,* are destitute of those transverse stri or lines observed by the microscope in ordinary muscular fibres. The chemical constitution of the middle coat of the arteries differs also from that of muscle, for it is less soluble in acetic acid, and more easily so in mineml acids, and it is believed by Ber zelius and Young not to contain the animal principle, fibrine, peculiar to muscular flesh. Although we fully admit the importance of these observations as establishing anatomical and chemical distinctions between muscular substance and the texture of the middle coat of the arteries, they do not appear to us to warrant the conclusion too hastily deduced from them by some, that this coat cannot be irritable, or does not possess any of the same properties as muscle, the existence or non-existence of which must be ascertained principally by physiologi cal evidence. For the transverse strim cannot be considered as characteristic of all muscular fibres; and were vve to reason in this way from the result of anatomical observations only, we should be necessitated to deny the irritability of various other textures, the con tractility of which from stimulation or without it, is universally admitted, although anatomists have not yet detected muscular fibres in them.