STUDY OF THE CIRCULATION. 153 



the fine adjustment of the microscope, in the first 

 place, longitudinal striae, which are produced by 

 wrinklings in the elastic layer of the internal coat ; 

 and in the second place, situated most internally, 

 elongated oval nuclei, which belong to the epithe- 

 lioid cells lining the vessel. Of course, two layers 

 of all these structures are come across in focussing 

 from above down. The outer coat is represented 

 merely by a few corpuscles and .fibres of connective 

 tissue which blend externally with the connective 

 tissue framework of the membrane. 



It will be found in carefully focussing from above 

 down, that at one position of the focus tlie small 

 vessel has exactly an appearance as if it had been 

 cut longitudinally through the middle, and as if the 

 top of the lower half were being examined. The 

 lumen is seen in the centre, with possibly a few 

 blood corpuscles still in it ; on either side of this a 

 well-marked line representing the inner coat ; out- 

 side this again what seems like a row of rounded 

 cells, which are really the encircling fibre-cells of 

 the muscular coat seen as if cut across; and finally, 

 here and there, outside of all, small cells presenting 

 the fusiform aspect of connective tissue corpuscles 

 seen in profile. All these appearances are exactly 

 the same as if a section had been made along the 

 vessel, and result from the fact that only those parts 

 of an object which lie in the horizontal plane that 

 happens to coincide with the focal distance of the 

 objective, are distinctly seen, so that it seems as if 

 only this particular slice tvere present. An " optical 

 longitudinal section" is thus obtained of the vessel. 



STUDY OF THE CIRCULATION. 



The study of the bloodvessels cannot be said to 

 be in any s.ense of the word complete until they have 

 been viewed in the living condition and with the 

 blood still moving through them. Such an observa- 

 tion can, of course, only be made whilst an animal 



