On Diapedesis. 79 



siderable portion of the field of the microscope ; and portions of it 

 had a close resemblance to the ramifying structure mentioned as 

 having been observed below the muscular layer (PI. LXX., Fig. 31). 



The multiplication, if it be justifiable, of these structural elements 

 in the other segments of the base which were not examined would 

 give a fair notion of the plesiform arrangement of the basal nervous 

 tissue. I presume that it consists of a reticulate structure beneath 

 the endothelium, which sends large branches between the vacuities 

 of the most delicate muscular layer, and which communicates with 

 a ramifying tissue in contact with the other muscular layers, and 

 that this ends in long fibres which supply the wide fibres of this 

 last-mentioned layer. 



The diffused nature of this nervous tissue is what might be 

 anticipated would be found in animals possessing such general 

 irritabihty of tissue, and probably its function is to assist in the 

 reflex movements of the animal, and to produce expansion of the 

 disk on the stimulus of hght. — Proceedings of the Eoyal Society, 

 vol. xxii., No. 151. 



IV. — On Diajjedesis : or the Passage of Blood-corpuscles through 

 the Walls of the Blood-vessels, and how to observe it. 



By Joseph Needham, F.K.M.S. 



The importance of the subject of diapedesis cannot be overrated, 

 for one can scarcely attend a course of lectures on physiology, 

 pathology, surgery, or medicine, without hearing, over and over 

 again, a description of, or a reference to, this process ; surely, then, 

 you will not regard . the time spent in verifying for yourselves so 

 important a fact as wasted. Before proceeding to the substance 

 proper of my discourse, 1 wish to observe that, although I may 

 draw now and again on physical facts, or encroach too much on 

 the domain of pathology, yet those digressions will only be made 

 to render the explanation of the subject more lucid, or to testify to 

 the importance of a right conception of it. 



We will, for convenience, consider our matter under four 

 headings. 



1. Mode of demonstration. 



2. Description of process. 



3. Explanation of phenomena. 



4. Concluding remarks. 



I. As to the modes of demonstration, various observers have 

 made use of difierent animals. The material has been drawn 

 chiefly from the Batrachians and their tadpoles. Fish have occa- 

 sionally been employed; recently warm-blooded animals have been 



