THE MOVEMENT OF THE BLOOD IN THE SMALLEST VESSELS. 179 



most important method, and it has been repeatedly employed by various 

 investigators since the time of Malpighi, who was the first to observe the 

 circulation of the blood in the pulmonary vessels of the frog. 



Method. Suitable objects for study with transmitted light are the tails of 

 tadpoles and young fishes; the web, the tongue, as well as the mesentery stretched 

 and secured by means of pins on a strip of wax pasted to the object- 

 carrier, or the lung of a curarized frog; in mammals the wing of the bat and the 

 nictitating membrane, drawn out from the orbit and spread out by means of 

 threads over a vertical glass slide ; and much less advantageously the mesentery. 



The following objects can be examined with a low power by reflected light: 

 the blood-vessels of the frog's liver, of the pia mater in the rabbit, of the frog's 

 skin, and of the mucous membrane on the inner aspect of the lip in human beings, 

 as well as of the palpebral and bulbar conjunctivas. 



With respect to the form and arrangement of the capillaries in the various 

 tissues, the following points are worthy of note: 



1. The diameter of the smallest vessels, which permits the passage of the 

 blood-corpuscles only in single file, may, however, vary from 2 to 5 //, and 

 in the larger vessels naturally permits the passage of several corpuscles 

 abreast. 



2. The Length is, on the average, about 0.5 millimeter; beyond this limit the 

 vessels either originate by the division of small arteries, or unite to form veins. 



3. The number of capillaries is extremely variable, being largest in tissues in 

 which metabolism is most active, as the lungs, the liver, and the muscles; and 

 smaller in others, like the sclera and the nerve-trunks. 



4. The presence of numerous anastomoses is particularly striking, with the 

 formation of plexuses, the shape of which depends principally on the form and 

 structure of the basal tissue. Thus, the capillaries are arranged simply in loops 

 in the papillae of the skin; as polygonal, retiform meshworks in the serous mem- 

 branes and on the surface of many glandular acini; as longitudinal tubes running 

 close together between the muscles and the nerve-fibers and between the straight 

 uriniferous tubules; in a radiating manner, converging to a central point in the 

 liver; and in the form of arcade-like loops at the free border of the iris and at 

 the corneo-scleral junction. 



With regard to the transition of the smallest arteries into the capillaries, a 

 distinction should be made as to whether the minute arterial twigs are end- 

 arteries that is, such as do not anastomose with other arterial twigs of the same 

 order, but break up directly into capillaries, and communicate with neighboring 

 arterial twigs only by means of capillaries; or whether before breaking up into 

 capillaries the neighboring arteries communicate by liberal anastomoses, large 

 enough to be called arterial. The presence or absence of arterial anastomoses 

 is important with respect to the nutrition of the region supplied by the vessels. 



In observing the blood-current itself it will be seen at once that the 

 red blood-cells progress only along the center of the vessel in the axial 

 stream, while the parietal, transparent layer of plasma remains entirely 

 free from them. The latter, designated Poiseuille's space, is recognizable 

 especially in the smallest arteries and veins, in which the axial stream 

 occupies three-fifths, and the light layer of plasma one-fifth, of the 

 entire width of the vessel. It is less distinct in the capillaries. Accord- 

 ing to Rud. Wagner, Poiseuille's space is wholly absent in the smallest 

 vessels of the lungs and the gills. The red blood-cells pass through the 

 smallest capillaries in single file. In larger vessels they move close to- 

 gether, frequently turning and twisting in their course. On the whole, 

 the rate of 'progress in the larger vessels is uniform; occasionally, how- 

 ever, as when there is a sharp bend in a vessel, the movement is at times 

 somewhat retarded, at times again accelerated. Wherever the stream 

 divides, a blood-cell occasionally remains attached to the projecting 

 ridge at the point of division, bending at its edges on each side into the 

 bifurcation of the capillary, and appearing somewhat thinned at the 

 center. Often it may adhere in this way for a long while, until, the cur- 



