3 



the injections of this celebrated man are still extant, and a few 

 microscopists in this country are fortunate possessors of some of 

 his beautiful preparations. 



The older anatomists, we are told,* with Haller at their head, 

 adopted a notion that arteries terminated in one of five ways: 1st, by 

 openings on the surface of membranes ; 2nd, in lymphatics ; 3rd, in 

 secreting canals ; 4th, in fat ; 5th, in veins. In those times the se- 

 cretion of mucus and fat could not be understood, without presup- 

 posing the existence of open extremities. This view of the subject 

 was strongly opposed by Mascagni, Prochaska, Soemmering, Hunter 

 and others, but there still remained a doubt whether there were not 

 some communication between the blood-vessels and the secreting 

 canals of the glands. In more modern times, the structure of the 

 secreting glands has been studied by Miiller and other physiologists 

 in process of development in the embrj^o, and the secreting canals of 

 the glands have also been injected, and no such communications 

 have been found to exist, the radicles of the secreting canals being 

 always closed at their radicle extremity. 



Anatomists now, with one consent, agree that in all organic tex- 

 tures, the transmission of the blood from the minute branches of the 

 arteries to the minute veins, is generally effected by the capillary net- 

 work, between the meshes of which the proper substance of the organ 

 lies ; this structure is commonly termed parenchyma, and speaking 

 of it as a tissue, it is termed parenchymatous tissue ; thus, for in- 

 stance, in a specimen of injected lung or kidney, or in a villus from 

 the intestine, the parts not occupied by blood-vessels constitute 

 the true parenchymatous tissue of the organ. All microscopists are 

 familiar with the circulation in the foot of the frog, and the tail of 

 the newt ; this is, for the most part, the capillary circulation. The 

 ramifications of the minute arteries form repeated anastomoses with 

 each other, and these anastomoses terminate at last in a continuous 

 net-work, from which the small branches of the veins take their rise. 

 The point at which the arteries terminate and the minute veins 

 commence, cannot be exactly defined ; the transition is gradual, but 

 the intermediate net-work is so far peculiar, that the small vessels 

 which compose it maintain nearly the same size throughout ; they do 

 not diminish in diameter in one direction, like arteries and veins, 

 hence the term capillary, from capillus, a hair. 



The size of the capillaries is proportioned in all animals to that 



* Miiller 's Physiology, translated by Baly, vol. i. p. 227. 



