MOTION OF THE BLOOD IN THE CAPILLARIES. 379 



tion to which they will be subjected, would seem to require some compensat- 

 ing power, in order that the perfect equality of pressure may be obtained, 

 which has been spoken of as existing in all parts of the arterial system. In 

 no other way than this, can the fibrous coat of the Arteries be regarded as 

 having any propulsive power over their contents ; except by a peristaltic or 

 vermicular movement, resembling that which takes place in the alimentary 

 canal ; and of such there is no evidence whatever. A very important use 

 may be assigned to this muscular coat, which has been generally overlooked 

 by physiologists, that of regulating the diameter of the tubes, in accordance 

 with the quantity of blood to be conducted through them to any part, which 

 will depend upon its peculiar circumstances at the time. Such local changes 

 are continually to be observed, in the various phases of normal life, as well as 

 in diseased states ; and they will be found to be constantly in harmony with 

 the particular condition of the processes of Nutrition, Secretion, &c., to which 

 the Capillary circulation ministers. Of this kind are the enlargement of the 

 trunks of the Uterine and Mammary arteries, at the epochs of pregnancy and 

 lactation ; the enlargement and strongly increased pulsation of the Radial 

 artery, when there is any active inflammation in the thumb ; the enormous 

 diameter which the Spermatic artery will attain, when the testicle is greatly 

 increased in size by diseased action ; and many other similar phenomena. In 

 such cases, it cannot be the action of the Heart that increases the calibre of 

 the vessels; since this is commonly unaltered, and is itself unable, as we have 

 just seen, even to maintain their permeability. It must, therefore, be by a 

 power inherent in themselves, that their dilatation is effected. The minute dis- 

 tribution of the Sympathetic nerve upon the walls of the arteries, -the known 

 power which this has of producing contractions, alike in their fibrous coat, and 

 in the muscular tunic of the intestinal canal, and various phenomena, which 

 indicate the power of certain states of mind over the dimensions of the arte- 

 ries, in particular parts of the body at least, render it highly probable, that 

 the calibre of the arteries is regulated in no inconsiderable degree through its 

 intervention.* The permanent dilatation, however, which is seen in the arte- 

 ries supplying parts that are undergoing enlargement, must be due, not to 

 simple dilatation merely, but to increased nutrition ; since we find that their 

 walls are thickened as well as extended. And, on the other side, when slow 

 contraction occurs in these tubes, as a consequence of disease, it must be in 

 part occasioned by atrophy ; since their nutrition is so much diminished, that 

 in time they almost entirely disappear, a portion of a large artery occasionally 

 shriveling into a ligamentous band. 



505. We now come to the last head of the inquiry into the powers which 

 convey the blood through the Capillary system ; that, namely, which con- 

 cerns the agencies existing in the capillaries themselves. Many discussions 

 on this subject may be found in physiological writings; and it has so imme- 

 diate a bearing on one of the most important questions in Pathology, the 

 nature of inflammation, that it deserves the fullest attention. The chief 

 question in debate, is the degree in which the Capillary circulation is influ- 

 enced by any other agency than the contractile power of the Heart and Arte- 

 rial system ; some physiologists maintaining, that this alone is sufficient to 

 account for all the phenomena of the Capillary circulation ; and others assert- 

 ing, that it is necessary to admit some supplementary force, which may be 

 exerted either to assist, retard, or regulate the flow of blood from the Arteries 

 into the Veins. We shall first inquire what evidence there is of the existence 



* For Anatomical evidence to this effect, see Henle on the Contractility of the Blood- 

 vessels, in Casper's Wochenschrift, May, 1840, and Brit, and For. Med. Rev., vol.x. p. 551. 



