320 CIRCULATION OF NUTRITIVE FLUID. 



and the ventricle are each divided by a partition, that is developed in 

 the middle of the heart; and thus the two auricles and the two ven- 

 tricles are formed. Whilst this is going on, a change takes place also 

 in the vessels that arise from the heart ; for the arterial trunk, that was 

 previously single, undergoes a division into two distinct tubes ; one of 

 which is connected with the left ventricle, and becomes the aorta, 

 whilst the other originates in the right ventricle, and becomes the pul- 

 monary artery. Of the four pairs of branchial arches, some are sub- 

 sequently obliterated ; whilst others undergo changes that end in their 

 becoming the arch of the aorta, the right and left pulmonary arteries, 

 and the right and left subclavians. 



567. The muscular power of the heart is much greater in the warm- 

 blooded than in the cold-blooded Yertebrata, in proportion to the 

 extent of the circulation which it is concerned in maintaining ; and it is 

 evidently destined to take a much larger share in the propulsion of the 

 fluid, than it is in the lower tribes. Many Physiologists indeed, are 

 of opinion that the movement of the blood is entirely due to the action 

 of the heart ; and this view appears to be supported by the results of 

 numerous experiments upon the circulation. But it is very difficult, if 

 not impossible, to make experiments that shall be really satisfactory 

 upon this point ; and it appears safer to trust to the " experiments 

 ready prepared for us by Nature," as Cuvier termed them, namely, 

 those lower forms of animated being, in which various diversities of 

 structure present themselves, and in which we can study the regular 

 and undisturbed effects of these. Thus we have seen that, in Plants 

 and the lowest Animals, which have no central impelling cavity, the 

 movement of the nutritive fluid is entirely dependent upon the power 

 that i& diffused through the network of vessels in which it circulates. 

 As we ascend the series, we find an organ of impulsion developed 

 upon a certain part of the vascular system, whose object it is to give 

 increased energy and regularity to the movement. And ascending still 

 higher, we find the moving power gradually concentrated, as it were, in 

 this organ ; yet it is not altogether withdrawn from the capillary net- 

 work, as we shall see from several facts to be presently adduced. The 

 particular actions of the Heart, the Arteries, the Capillaries, and the 

 Veins, will now be considered in more detail. 



3. Action of the Heart. 



568. The Heart is a hollow muscle, endowed in an eminent degree 

 with the property of irritability ; by which is meant, the capability of 

 being easily excited to movements of contraction alternating with relaxa- 

 tion ( 347). At first sight, its actions seem different from that of the 

 muscles which are called into action by the impulse of the will ; for in 

 these there is apparently no such alternation, the state of contraction 

 being kept up as long as the will operates. But it has been already 

 explained that, even in these, the individual fibres are probably in a 

 state of continual alternation of contraction and relaxation, during their 

 active condition, one set taking up the action, whilst another is return- 

 ing to the state of relaxation. Hence the chief peculiarity in the Heart's 



