CAPILLARY CIRCULATION. 187 



4. They form numerous anastomoses, and give rise to net-works, whose form and 

 arrangement are largely determined by the arrangement of the tissue elements them- 

 selves. They form simple loops in the skin, and polygonal net- works in the serous 

 membranes, and on the surface of many gland tubes ; they occur in the form of 

 elongated net- works, with short connecting branches in muscle and nerve, as well 

 as between the straight tubules of the kidney ; they converge radially towards a 

 central point in the lobules of the liver, and form arches in the free margins of the 

 iris, and on the limit of the sclerotic and cornea. 



[A good contrast as to the vascularity of two adjacent parts is seen in the gray 

 and white matter of the brain, the former being very vascular, the latter but slightly 

 so.] 



[Direct termination Of Arteries in Veins. Arteries sometimes terminate 

 directly in veins, without the intervention of capillaries, e.g., in the ear of the 

 rabbit, in the terminal phalanges of the fingers and toes in man and some animals, 

 in the cavernous tissue of the penis (Hoyer). They may be regarded as secondary 

 channels which protect the circulation of adjacent parts, and they may also be 

 related to the heat-regulating mechanisms of peripheral parts (Hoyer).] 



End-Arteries. In connection with the termination of arteries in capillaries, 

 it is important to determine if the arterioles are " end or terminal arteries," i.e., if 

 they do not form any further anastomoses with other similar arterioles, but 

 terminate directly in capillaries, and thus only communicate by capillaries with 

 neighbouring arterioles or the arteries may anastomose with other arteries just 

 before they break up into capillaries. This distinction is important in connection 

 with the nutrition of parts supplied by such arteries (Cohnheim). 



Capillary Circulation. On observing the capillary circulation, we 

 notice that the red corpuscles move only in the axis of the current 

 (axial current), while the lateral transparent plasma- current flowing on 

 each side of this central thread is free from these corpuscles. [The 

 axial current is the more rapid.] This plasma layer or " Poiseuille's 

 space " is seen in the smallest arteries and veins, where f is taken up 

 with the axial current, and the plasma layer occupies J on each side of it 

 (Fig. 85). A great many, but not all, of the colourless corpuscles run in 

 this layer. It is much less distinct in the capillaries. Eud. Wagner stated 

 that it is absent in the finest vessels of the lung and gills [although 

 Gunning was unable to confirm this statement.] The coloured corpuscles 

 move in the smallest capillaries in single file one after the other ; in 

 the larger vessels, several corpuscles may move abreast, with a gliding 

 motion, and in their course they may turn over and even be twisted 

 if any obstruction is offered to the blood-stream. As a general rule, in 

 these vessels the movement is uniform, but at a sharp bend of the 

 vessel it may partly be retarded and partly accelerated. Where a 

 vessel divides, not unfrequently a corpuscle remains upon the projecting 

 angle of the division, and is doubled over it so that its ends project 

 into the two branches of the tube. There it may remain for a time, 

 until it is dislodged, when it soon regains its original form on account 

 of its elasticity. Not unfrequently we see a red corpuscle becoming 

 bent where two vessels meet, but on all occasions it rapidly regains 



