370 THE CIRCULATION OF THE BLOOD. [CHAP, xxvin. 



force of the blood in the veins diminishes or increases with that in 

 the corresponding artery under certain circumstances. 



3. In Fishes, the whole blood ejected from the heart passes, 

 from the bulbus aortas, through the branchial capillaries, before it 

 enters the systemic vessels ; thus illustrating how the heart's force 

 may be propagated through a complex network of minute capil- 

 laries, to both arteries and veins, as well as to the system of 

 capillaries which intervenes between them. 



4. In debilitated animals, it is evident, from the jerking move- 

 ment of the blood in the capillaries, corresponding with the action 

 of the heart, that the impulse of that organ is extended to these 

 vessels, unbroken by the elastic reaction or the muscular contrac- 

 tion of the arteries. This may be well seen in watching the circu- 

 lation in the frog's web, or in the tail of the tadpole. 



Thus, there can be no doubt that the heart's force is not only 

 fully adequate to, but is the principal agent in, the maintenance of 

 the capillary circulation. When this force fails, the circulation in the 

 capillaries suffers as much as, if not more than, that in any other 

 portion of the vascular system ; and the sluggish transmission of 

 the blood through the capillaries, such as we often find when the 

 heart is simply weakened, occasions congestions, particularly of 

 dependent parts, and then, by the filtration of the serous por- 

 tion of the blood through the parietes of the vessels, oedema and 

 anasarca. 



But however readily we may concede that the heart's action is 

 the principal force, it must be confessed there are certain phe- 

 nomena which do not admit of satisfactory explanation, on the 

 supposition that it is the only one employed in the maintenance 

 of the capillary circulation; and it seems more reasonable to 

 assume the influence and exercise of some other force super- 

 added to this, in order to explain various phenomena which take 

 place in, or which are dependent on, the circulation through the 

 capillaries. 



The more remarkable of these phenomena are blushing, or, in 

 more general terms, the influence of mental emotion upon the ca- 

 pillary circulation ; the influence of local irritation, whether acci- 

 dental or morbid ; and the effects of asphyxia. 



It seems highly probable that in the ordinary molecular changes 

 which take place in the nutrition of the tissues, a force is generated, 

 which, in its normal state, must promote, by an attractive influence, 

 the flow of blood through the capillaries. The cessation of such a 

 force would operate unfavourably to the flow of blood through the 



