160 THE BLOOD AND ITS CIRCULATION. 



was, according to them, one of these organs of peripheral 

 impulse, intended to aid the action of the heart. We can 

 easily see, by our study of the circulation, that the contrac- 

 tion of the capillaries, the so-called accessory hearts, would be 

 rather an obstacle than an assistance to the flow of the blood. 

 The pulsations felt in an inflamed tissue (in a whitlow, for 

 instance) were adduced as a proof of the rhythmical con- 

 traction of the capillaries, but we have already explained 

 this sensation as being caused by a paralytic dilatation of 

 the smaller arteries. We have also seen that the effect of 

 hemostatic agents is to produce the contraction, not of the 

 capillaries, but of the small arterial vessels. The so-called con- 

 tractility of the capillaries thus belongs entirely to the region 

 of theory, and rests on no positive fact ; and the experiments 

 made on the mesentery of a frog have reference to contrac- 

 tion of the small arteries, and not of the capillaries. 



The capillaries, as we have considered them, form a per- 

 fectly well-defined part of the circulating system, and their 

 physiological properties are quite distinct from those of the 

 arteries and the veins: we consider as capillaries, with Kol- 

 liker and C. Morel, only those small vessels, which, without 

 undergoing any previous preparation, appear as tubes of an 

 amorphous substance in which oval nuclei are inserted. 

 Some histologists, however, Henle and Charles Robin in 

 particular, class under this denomination both the capillaries 

 properly so called, and the finest ramifications of the small 

 arteries and veins. Thus Ch. Robin divides the capillaries 

 into three kinds : 1, capillaries properly so called, distin- 

 guished by having a single homogeneous tunic with a 

 nucleus, their diameter being from -^faa of a millimetre (the 

 diameter of a blood globule) to T f^ of a millimetre; 2, 

 capillaries of the second kind, having a diameter of from 

 TOGO to TpoiT f a millimetre, and provided with a double 

 coat, the inner one being a continuation of the outer, which 

 is formed of contractile cellular fibres arranged in circles ; 

 3, capillaries of the third kind, their diameter being from 

 T ^ to TO\% and having, beside those already mentioned, a 

 third external tunic formed of connective tissue. For the 

 physiologist, these two latter kinds of vessels are evidently 

 small arteries and veins, likewise possessing great contrac- 

 tility ; they represent, exactly, the base of the arterial and 

 venous cone, which abounds in smooth muscular elements, 

 to the exclusion of the elastic element. 



The structure of the capillaries, properly so called, is not, 



