of the blood corpuscles ; thus amongst the Reptilia, where the blood- 

 corpuscles are the largest, the capillaries are also the largest, but it 

 does not follow that they should be always of the same size in all the 

 tissues of one and the same animal, for if we examine and carefully 

 measure in the human subject their sizes in different tissues, we shall 

 find that they vary greatly even in individual tissues, and at a rough 

 estimate, examples may occur as large as a thousandth, whilst others 

 are as small as the four-thousandth or five -thousandth of an inch. 

 They should be measured if possible in their natural state ; when in- 

 jected, their size is slightly increased, but when dried they diminish 

 so considerably, that in some specimens vessels imperfectly filled with 

 injection have been known to shrink from the three-thousandth to the 

 twenty-thousandth of an inch. 



In foetal animals, having the blood corpuscles larger than those in 

 the adult state, the capillaries also are larger. 



Capillaries are, with very few exceptions, always supported by an 

 areolar net-work, which serves not only as an investment to them, 

 but connects them intimately with the tissues they are destined to 

 supply. 



Having said thus much of the nature of the capillaries, we will 

 now proceed to consider those points which are the more immediate 

 objects of this paper ; viz., the distribution of the capillary system in 

 the respiratory organs of Fishes. 



In the majority of osseous fishes there are four gills on each side, 

 supported by long curved branchial arches. Each gill consists either 

 of a single or a double series of lancet-shaped laminae attached to 

 the branchial arch, something like the teeth to the back of a comb. 

 When there is only one series of laminae, the gill is termed uniserial, 

 but when two, biserial. The branchial arches in osseous fishes are 

 more or less bony, and from the lower convex surface are given off 

 in uniserial gills one, in biserial two, elastic, more or less round fila- 

 ments, which form the outer support or framework of the laminae ; 

 and if the mucous membrane and all the soft parts be removed, it 

 will be found that in many fishes a series of more minute filaments 

 are given off at right-angles from the inner side of the larger one, 

 each of these smaller filaments serving not only to support the 

 laminae, but each one performing the same office to the outer margin 

 of the folds of mucous membrane on the laminae, as the larger fila- 

 ment does to the lamina itself; this skeleton of the lamina resem- 

 bles in every respect a small comb. Upon this framework we have 

 a fibrous or areolar tissue, which not only invests the little teeth, but 



