RESPIRATION. 



289 



arc placed, like the leaves of a book, in close 

 apposition with each other, the unattached 

 sides floating freely in the water, the current 

 of which, as it traverses the branchial pas- 

 sages, bearing directly on the flat surfaces or 

 the leaflets, separates them effectually from 

 each other. Viewed edgewise, as represented 

 in the following sketch, the arrangement of 

 these respiratory membranous extensions may 

 be more fully understood. On the branchial 

 processes of the eel these leaflets amount to 

 700 in number ; on those of the turbot, to 

 900 ; on those of the cod, to 1000; on those 

 of the salmon, to 1-400. Especial attention 

 is invited to Jig. 237. It shows that only 

 the mucous membrane (/. c. the layer of 

 epithelium and basement membrane) is 

 doubled upon itself, so as to assume the 

 form of folds, the stratum of capillary blood 

 being single. From this beautiful arrange- 

 ment it results that the blood, in its passage 

 through the labyrinth of this plexus, must 

 present two sides of an extremely divided 

 stream to the agency of the circumfluent 

 medium. If the network of vessels were 

 duplicated upon a supporting basis, only 

 one side of the sheet of blood could re- 

 ceive the influence of the surrounding me- 

 dium. Regarded mechanically, such a plan 

 would present little of the delicacy and per- 

 fection which really distinguishes this most 

 elaborate specimen of organised structure. 

 This singlc-\&\er disposition of the respiratory 

 vessels, doubling thus the surface of exposure, 

 obtains as the universally prevailing type of 

 structure in the breathing organs of aquatic 

 animals. It is an arrangement which faci- 

 litates in a very remarkable manner the inter- 

 change of gases between the blood and the 

 water. The capillary vessels of the branchiae 

 of fishes, in internal diameter, exceed very 

 little the long axis of the blood-corpuscle. 

 The internal calibre of these channels varies 

 from -g^Q to T i Q of an inch ; the blood-cor- 

 puscle in the cod measure in the long dia- 

 meter Y-jVr inch, and the channels forming 

 the capillary network present in all parts 

 precisely the same calibre. 



Cartilage, or Supporting Si/stem of the 

 SranchitE. The skeletons of fishes are struc- 

 turally distinguishable into three classes ; 

 the osseous, fibro-cartilaginous, and true car- 

 tilaginous. In the conventional language of 

 comparative anatomists the last is described 

 as the least completely organised, and the 

 first the most. This distinction, however, 

 which obtains in the adopted nomenclature 

 of science, has no counterpart in that portion 

 of the skeleton which sustains the foliage of 

 the branchiae. In the pctromi/~on fluviulis 

 the wicker-work of cartilage which Miiller 

 has called the cartilaginous basket of the 

 branchiae, and which Prof. Owen regards as 

 homologous with the epibranchial system of 

 osseous fishes, detaches processes of almost 

 membranous tenuity stretching inwards to 

 supply a mechanical support to the slender 

 respiratory foliage. In the myxinoid families, 

 cartilage and calcigerous structures are sub- 



stituted, in various parts of the body, by 

 ehstic fibres. In the osseous fishes the 



Fig. 237. 



Skeletal framework of the lamella; of the gills in. Fishes 



( Original^) 



a, section of the branchial arch, which, in osseous 

 fishes, consists of well-marked bone, and in the car- 

 tilaginous, of soft cartilage ; b, marks the junction 

 between the system of the arch and that of the 

 lamellae, proving that the latter is quite distinct 

 from the former ; c, illustrates that portion of the 

 framework which occupies the obtuse border of the 

 lamella, exhibiting its external thick marginal 

 fluted channel destined to convey the branchial 

 artery from the base to the apex of the lamella) 

 d, denotes that part of the skeletal fabric which 

 coincides in situation with the acute margin of the 

 lamella ; e, marks the situation of the branchial vein 

 as it emerges from the innermost aspect of the axis 

 supporting the lateral processes f, which, by their 

 elasticity, keep open or stretched the true mem- 

 branous respiratory leaflets. 



branchial skeleton presents itself under the 

 most readily distinguishable characters. 

 From this class, accordingly, the following 

 illustrative details will be drawn. The 

 respiratory segment of the skeleton com- 

 prises the hyoid system and its dependen- 

 cies, in which are included the branchio- 

 stegal rays and the branchial arches, on which, 

 finally, is elaborated an exquisite arrange- 

 ment of solid points upon which important 

 mechanical functions devolve, in the move- 

 ments of the apparatus of respiration.* The 



* As it is the design of this paper to refer no 

 more to the descriptive departments of comparative 

 anatomy than is found indispensable to elucidate 

 questions of ultimate structure, I must refer the 

 reader, for the solution of the homological and tele- 

 ological details on this subject, to the work of Prof. 



u 



