PHENOMENA OF THE CAPILLAPwY CIRCULATION. 87 



parent layer was observed by Malpighi, Ilaller, and all who have described the capillary 

 circulation. Poiseuille recognized its true relation to the blood-current and explained 

 the phenomenon of the still layer by physical laws, which had been previously estab- 

 lished with regard to the flow of liquids in tubes of the diameter of from one-twenty- 

 fifth to one- eighth of an inch, but which he had succeeded in applying to tubes of the 

 size of the capillaries. 



A red corpuscle occasionally becomes involved in the still layer, when it moves 

 slowly, turning over and over, or even remains stationary for a time, until it is taken up 

 again and carried along with the central current. A few white corpuscles are con- 

 stantly seen in this layer. They move along slowly and apparently have a tendency to 

 adhere to the walls of the vessel. This is due to the adhesive character of the surface of 

 the white corpuscles as compared with the red, which can easily be observed in examin- 

 ing a drop of blood between glass surfaces, the red corpuscles moving about with great 

 facility, while the white have a tendency to adhere. 



Great differences exist in the character of the flow of blood in the three varieties of 

 vessels which are under observation. In the arterioles, which may be distinguished from 

 the capillaries by their size and the presence of the muscular and fibrous coats, the move- 

 ment is distinctly remittent, even in their most minute ramifications. The blood moves 

 in them with much greater rapidity than in either the capillaries or veins. They become 

 smaller as they branch, and carry the blood always in the direction of the capillaries. 

 The veins, which are relatively larger than the arteries, carry the blood more slowly 

 and in a continuous stream from the capillaries toward the heart. In both the arteries 

 and veins, the current is frequently so rapid that the form of the corpuscles cannot be 

 distinguished. Only a portion of the white corpuscles occupy the still layer, the rest 

 being carried on in the central current. 



The circulation in the true capillaries is sui generis. Here the blood is distributed in 

 every direction, in vessels of nearly uniform diameter. The vessels are generally so small 

 as to admit but a single row of corpuscles, which move almost like beings endowed with 

 volition. In a single vessel, a line of corpuscles may be seen moving in one direction at 

 one moment, a few moments after, taking a directly opposite course. Spallanzani, 

 in one of his observations, describes the following phenomenon : Two single rows of 

 corpuscles, passing in two capillary vessels of equal size, were directed toward a third 

 capillary vessel, formed by the union of the two others, which would itself admit but a 

 single corpuscle. The corpuscles in one of these vessels seemed to hold back until those 

 from the other had passed in, when they followed in their turn. When the circulation is 

 normal, the movement in the capillaries is always quite slow as compared with the move- 

 ment in the arterioles, and is continuous. Here, at last, the intermittent impulse of the 

 heart is lost. The corpuscles do not necessarily circulate in all the capillaries which are 

 in the field of view. Certain vessels may not receive a corpuscle for some time, but, after 

 a while, one or two corpuscles become engaged in them and a current is finally established. 



Many interesting little points may be noticed in examining the circulation for a suffi- 

 cient length of time. A corpuscle is frequently seen caught at the angle where a vessel 

 divides into two, remaining fixed for a time, distorted and bent by the force of the cur- 

 rent. It soon becomes released, and, as it enters the vessel, it regains its original form. 

 In some of the vessels of smallest size, the corpuscles are slightly deformed as they pass 

 through. The scene is changed with every different part which is examined. In the 

 tongue, in addition to the arterioles and venules with the rich net-work of capillaries, 

 dark-bordered nerve-fibres, striated muscular fibres, and pavement-epithelium can be 

 distinguished. In the lungs, the view is very beautiful. Large, polygonal air-cells are 

 observed, bounded by capillary vessels, in which the corpuscles move with extreme 

 rapidity. It has been observed that the larger vessels are crowded to their utmost capa- 

 city with corpuscles, leaving no still layer next the walls, such as is seen in the circula- 

 tion in other situations. 



