154 THE BLOOD AND ITS CIRCULATION. 



that of a hollow cylinder ; the pressure of the atmosphere, 

 however, prevents it from dilating and assuming this form, 

 and it can only completely assume it when the air is admitted 

 into its cavity by an incision. 



Thus, during life, the arteries are in a state of permanent 

 tension : this constitutes their tonicity, and is similar to what 

 we have already studied in the sphincters, and in the mus- 

 cles in general. 1 The effect of this peculiar condition is that 

 the arteries do not serve simply to conduct the blood ; they 

 transform the circulation, changing the intermittent stream 

 of blood which gushes from the heart, into a continuous flow. 

 In the large arteries near the heart, the flow of the blood is still 

 intermittent, but as we proceed farther into the arterial sys- 

 tem, we find that it becomes continuous. Indeed, by de- 

 ducing from the flow of the carotid artery that of the 

 origin of the aorta, it has been calculated that each blood 

 wave contains about 180 grammes of blood. This enormous 



1 These considerations as to the natural and the apparent form 

 of an organ, of a simple tissue, or one composed of several ele- 

 ments, are of the highest interest in general physiology, and 

 throw sometimes unexpected light upon the explanation of certain 

 phenomena. We have already studied the muscle under two phy- 

 siological forms (form No. 1 and No. 2) which they scarcely ever 

 perfectly attain. There are certain ligaments, as the yellow liga-- 

 ments of the vertebral column, which, also, scarcely ever attain 

 their natural form. If the series of spinous processes and laminae 

 be divided from the series of the articulating masses by two strokes 

 of a saw behind and throughout the length of the vertebral laminae, 

 and after this separation the length of the two vertical halves of the 

 column be compared, we shall find that the back part has shortened 

 in a very remarkable manner, the shortening corresponding nearly 

 with the height of three vertebrae of medium size. The yellow 

 ligaments are evidently the cause of this shortening; they are re- 

 strained by the separation and the rigidity of the laminae upon 

 which they are stretched, which prevents their assuming their 

 natural form, and they can return to it only on the withdrawal of 

 this antagonistic force. 



We shall see that the natural form of the lung in the living body 

 differs from the natural form of the lung in the dead body, and 

 that in the living and normal organism the first is never found 

 perfectly developed. This study will help us to comprehend easily 

 the mechanism of expiration. 



By the natural form, either of a tissue or an organ, must be under- 

 stood the form peculiar to the tissue or organ, independent of all foreign 

 influences, more or less constant, which have a tendency to antagonize 

 or oppose its peculiar form. 



