240 CIRCULATION OF THE BLOOD. 



cannot be discerned, but in the veins both colored and color- 

 less corpuscles can be distinguished ; and it is soon noticeable 

 that, while the former are confined to the axial current, the 

 latter show a tendency to loiter along the inner surface of the 

 vessel, like round pebbles in a shallow but rapid stream. The 

 observation may be continued without material change for 

 many hours ; but if aiw artery is measured from time to time 

 micrometrically, it will be found that after a while it becomes 

 wider. (Tn this dilatation of the arteries follows a correspond- 

 ing though less marked enlargement of the veins, and, if the 

 attention of the observer is fixed upon these last, it is seen 

 that the circulation, which was before so active, undergoes a 

 marked and almost sudden slowing. This slowing indicates 

 that the membrane, in consequence of its exposure to the air, 

 is becoming inflamed; simultaneously with it, the colorless 

 corpuscles, instead of loitering here -and there at the edge of 

 the axial current, crowd in numbers against the venous walls. 

 In this way the vessel becomes lined with a continuous pave- 

 ment of these bodies, which remain almost motionless, not- 

 withstanding that the axial current still sweeps by them, 

 though with abated velocity. If, at this moment the atten- 

 tion is directed to the outer contour of the vessel, it is seen 

 that minute, colorless, button-shaped elevations spring from it, 

 each of which first assumes the form of a hemispherical pro- 

 jection, and is eventual!}' converted into a pear-shaped body, 

 attached by a stalk to the outer surface of the vein. This 

 bod}', which has thus made its way through the vascular mem- 

 brane, is, I need scarcely sa} r , an amoeboid colorless corpuscle. 

 It soon shows itself to be so by throwing out delicate prongs 

 of transparent protoplasm from its surface, especially in the 

 direction from which it has come. 



The methods to be employed for the study of the circulation 

 in the tongue and in the web are fully described in Chapter 

 VII. For investigations relating to the innervation and con- 

 tractile movements of the smallest arteries, the tongue is of 

 little value, though superior to the mesentery and web for the 

 study of inflammation. The web, on the other hand, is pre- 

 ferable, for the purposes first mentioned, to either the tongue 

 or mesentery. 



45. Capillary Circulation in Mammalia. The study 

 of the capillary circulation of mammalia under the micro- 

 scope is attended with great difficulty in the first place, be- 

 cause (if we except the wing of the bat) there is no exteriial 

 part sufficiently transparent for observation under high power; 

 and, secondly, because if internal parts are used, the injurious 

 effects of exposure are much greater than those which occur 

 in batrachians. To overcome these difficulties it is necessary 



