Minute Circulation of the Branchi.— The branchial arteries proceed on either side, symmetrica ly, from the aorta, and travel through a groove along the convex border of the branchial arches, the veins lying to the outside of the artery, that is, next to the pec tinated fringes of the gills. The cartilage system of the arches and that of the penknife shaped processes (bearing the ultimate bron chial leaflets) are quite distinct and uncon nected save by fibrous structure. It is in the in tervals between these solid parts that the trunks of the vessels are disposed. As the pectinated processes (a, fig. 235.) arise, when biserial, alternately from the arch, the arterial branches leading to them observe a similar arrange ment. The arteries carrying venous blood invariably run along the thick border or outer margin of each process, the vein occupying the innermost edge. A branchial process, attached at right angles to the convex border of the arch, resembles a penknife, the back of which corresponds with the thick and outermost borders, and the edge of the blade with the thin or acute side of the process. A string carried from base to point would mark the position of the branchial artery convey ing venous blood : brought back along the acute edge, it would denote the line of the branchial vein bearing arterial blood. In the marsupio-branchii (as the myxine, lam prey, &c.), in which the gills are fixed and inoperculate, this is also the virtual arrange ment of the secondary' trunks. In the lopho branchii (sea-horse, pipe-fish, &c.), in which the branchim are tufted, the disposition of the minute vessels is not dissimilar. The fiat surfaces of the penknife-shaped process are gorgeously festooned by foliaceous multi plications of the membrane (fig. 235, b). In the annexed illustration, one of these leaflets has been shown. In length it does not coincide with the width of the process; the secondary trunks, passing through the substance of the cartilage (b) at right angles with the primary (a) are longer than the corresponding se condary veins on the opposite side of the bladelet ; hence the one-sided position of the leaflet (c) bearing the capillary netzvork. These membranous processes, by which the active breathing surface is so extensively multiplied, are 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 meinbranous 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 1400. Especial attention is invited to .fig. 237. It shows that only the mucous membrane (i. e. the layer of epithelium and basement membrane) is doubkd upon itself, so as to assume the form of folds, the stratum of capillmy 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 single-layer 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 branchix of fishes, in internal diameter, exceed very little the long axis of the blood-corpusele. The internal calibre of these channels varies from rim to 4. of an inch ; the blood-cor pii.cle in the cod measure in the long dia meter re4-7 111C h and the channels forming the capillary network present in all parts precisely the same calibre.
Cartilage, or Supporting Systenz of the Bronchia—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 branchiw. In the petromyzon the wicker-work of cartilage which Miller has called the cartilaginous basket of the branchire, 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 elastic fibres. In the osseous fishes the 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 vvhich 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 gill-bearing arches are not composed of single undivided curved bones, but of several ele ments, adjusted with express reference to the elasticity and flexibility of the whole. Six of these arches are primarily developed, and five permanently retained. The first four support gills, the fifth is beset with teeth which guard the opening of the gullet : this latter is termed the pharyngeal arch ; the rest the branchial arches. From the convex side of the branchial arches a double series of interlocking penknife-shaped processes radiate. On the flat surfaces of these pro cesses a gorgeous arrangement of mem branous leaflets is disposed in a transverse manner, each leaflet standing, as already de scribed,on its edge (fig. 237.). This rich foliage, bearing the complex web of the respiratory capillaries, is itself sustained by a machinery of elastic solid parts, hitherto unknown in comparative anatomy.
The series of curvilinear bones denomin nated the arches of the branchim, and exhi bited in section at a, fig 237., are inferiorly attached to the sternal chain of bones, pro ceeding upwards and backwards, and describ ing a curve, which in different genera varies in degrees of sharpness, and which finally affix themselves by means of ligaments to the base of the cranial bones. These curved bones are constructed of several separable pieces, adjusted with artful reference to the resilient properties of the curvilinear figure. The act of opening the mouth in the fish deter mines a consecutive series of movements, which end in the preparation of the gills to be traversed by the branchial current. By an appropriate intervening mechanism the move ments of protruding and retracting the mouth occasion irrespectively the approximation and separation of the branchial processes, accom panied by an alternate increase and decrease of the curvature of the sustaining arch, The straight penknife-shaped processes (1, 2. fig. 239. x.), as stated, diverge from the convex ities of the branchial arches. In nearly ail the osseous fishes, these processes form a double series, the gills being accordingly dis tinguished as biserial. In those genera in which these processes form a single line the gills are said to be uniserial. In the genera cottus (bull-heads), labroicke (rock-fish or wrasses), sebastes, (Norway haddock), scorpmnidm (hog-fish), Teidm (John Dory-, &c.), the nilotic-fishes, polipterus, gobroidm (blennies and gobies), lepadogastithe, a genus of small sucker-fishes), and the cyclopteridm or lump-fishes, may be ranged under one great group, characterised by three biserial and one uniserial gill. The genera sophiidm (angler), batrachoidm, the tetraodons (or globe-fish), the diodons (commonly called the sea porcu pines), and that curious genus of monop terns found on the seas of the Molluccas, the gill opening in which is united by transverse partitions of membrane, are characterised by three symmetrical biserial gills. Other less regular genera present other diversities in the disposition of the branchial processes The penknife-shaped processes (fig. 237.), which radiate from the convexities of the bronchial arches are bony in the osseous fishes; but only that part of the solid basis of the process is bony which corresponds with and forms the substance of its blunt or thick margin. In the cartilaginous orders this por tion is composed of cartilage, the component cells as well as the outline of which are very dissimilar to those of the former class. As in dicated in the above illustration, that piece in the skeleton of the branchial process which forms the obtuse border exhibits a groove seen sectionally at (b,b, fig. 237.), serving for the lodgement of the processal branch (g) of the branchial artery. The internal edge of this bony piece is dentated by pro jecting spines, adapted perfectly to support the soft structures. This part is represented transversely in fig. 238, in which also the disposition of the constituent cells may be remarked to bear advantageously upon the mechanical functions of the parts. As regards the arrangement of these cells, this piece in the framework of the gill-process may be- divided into two parts, by a longitudinal line, on t he outer side of which the long axes of the cells, which are oblong, are directed transversely with respect to the inner : the axes of the cells are parallel with that of the processes of which they are thc constituents parts. This arrangement is most distinctly marked in those genera in which the skeleton is little calcified. Functionally considered the piece occupying the blunt edge of the process de termines its penknife-like contour, and confers strength and straightness of direction, thus favouring the contact between the respiratory foliage and the surrounding medium. It dis charges the passive office of sustaining the soft structures and of extending the plane superficies over which the branchial blood vessels are distributed.