382 MANUAL OF HISTOLOGY. 



Finally, the capillaries, supplied likewise, it would appear, with nerves, 

 constitute the most important part of the whole vascular system. Through 

 the membranes of which they are formed takes place the interchange of 

 matter between the plasma of the blood and the organic juices : from them 

 are exuded those fluids which .appear afterwards as glandular secretions. 

 We have already learned ( 205) that upon the richness in capillaries of 

 any organ or tissue depends the energy of its transformative power. 

 The variety in the matters given out from and received into the several 

 portions of the capillary system, may partly depend upon a different 

 molecular constitution of the walls of the vessels ; partly on difference in 

 composition of the blood of various regions of the vascular system, as well 

 as the changeable nature of the organic fluids. The form of the afferent 

 and efferent vessels of capillary networks is also certainly of importance. 

 It will suffice here to point to the retarding effect of the glomerulus of 

 the kidney (fig. 370) on the stream of blood traversing the organ. The 

 different amount of pressure in the several parts of the capillary system 

 produced by this is, however, probably the most important moment in 

 the processes taking place here. 



We have already referred ( 81) to an occurrence of great vital import- 

 ance, which has only been recognised very recently, namely, the passage 

 of colourless as well as red blood corpuscles through the uninjured walls 

 of vessels, the contractility of the vascular cells appearing to provide for 

 the closure of each successively formed aperture. 



We turn now to the question, so frequently discussed, as to the existence 

 of the so-called vasa serosa, or plasmatic vessels. Are there in the system 

 capillaries of such small calibre that they do not admit, in the normal 

 state, of the passage of blood-cells through them, and consequently only 

 serve for the transmission of the liquor sanguinis ? Such vessels do not 

 exist, although it was formerly supposed that such was the case, and that 

 a non-vascular organ could rapidly acquire capillaries when in a state of 

 irritation by the enlargement of these to such an extent as to allow of the 

 passage of blood-cells. Long ago attention was directed by Henle to the 

 presence in the substance of the brain of fine filiform tubes communi- 

 cating with ordinary capillaries. These were afterwards discovered to be 

 merely fine vessels which had been unnaturally stretched, and thus 

 narrowed ( Welcker). Here and there efforts have, indeed, been made to 

 maintain the transition of capillaries into plasmatic passages or " sap- 

 canals" (Coccius, Eckard, Heidenhain), but without success. However 

 the hypothesis of such an intermediate system of vessels between capil- 

 laries and lymphatic radicles may recommend itself on account of its con- 

 venience, observation does not support anything of the kind. 



210. 



The circulation of the blood in the living body is one of the most 

 beautiful spectacles which the microscope can reveal. The readiest mode 

 of seeing this is to examine the transparent parts of cold-blooded verte- 

 brates, such as the web of the frog's foot or the tail of the tadpole. The 

 embryos of fishes and birds, the wing of the bat, the mesentery of pre- 

 viously narcotised small mammals, &c., will serve the same purpose. 



Taking, for instance, the first-named portion of the frog's body (fig. 3S3-), 

 we see in the larger arterial and venous branches of the web the blood 

 streams coursing in opposite directions, with a rapidity magnified natu- 



