RESPIRATION. 



281 



In both they are simple loops, but a distinc- 

 tion is obvious ; so evident as to render it 

 impossible that the transition from the ex- 

 posed to the concealed state of the branchiae 

 can consist in a bodily retractation. On the 

 external organs, preparatorily to their disap- 

 pearance, the vibratile cilia first cease, the 

 epidermis then increases in density, the meshes 

 between the blood-capillaries enlarge, and the 

 vessels become obliterated. These declining 

 changes are not limited to the extreme distal 

 ends of the branchial lobules. They occur 

 simultaneously on every part of the surface. 

 Temporary arches (r) of delicate cartilage not- 

 arise within the branchial chamber. It is from 

 the convexities of these arches after the manner 

 of the pisciform type that the neiv vessels (a) 

 of the internal temporary gills proceed. They 

 are appended under the character of de- 

 licate flocculi. Enlarged, they appear as 

 minute digitations. Each carries a looped 

 vessel, and is loosely invested with a de- 

 licate membrane (cl). This membrane belongs 

 to the mucous, not to the epidermal, class ; 

 and yet it differs in a striking manner from 

 that which lines the rest of the branchial 

 chamber. Nowhere is it ciliated. That co- 

 vering the branchial vessels is remarkably 

 thinner than the parietal portion. The 'former, 

 however, is true epithelium. Its constituent 

 scales are distinctly traceable by their out- 

 lines, though they are as structureless as a 

 basement membrane. It is not often that it 

 happens that the epithelium of a breathing 

 organ overlies, as in this instance, perfectly 

 homogeneous parts. Nothing but the proper 

 coats of the vessel lie underneath. They are 

 literally structureless and hyaline. The cells 

 of the superficial epithelium, therefore, admit 

 of indisputable definition. It is not " base- 

 ment membrane," but epithelium, though 

 attenuated, that here invests the respiratory 

 vessels. By this demonstration a principle is 

 established. Epithelium is not supplanted by 

 any other structure on the organs dedicated 

 to respiration. No other instance, however, 

 is known within the limits of the vertebrate 

 kingdom in which this epithelium is ciliated, 

 than that afforded in the case of the tem- 

 porary external gills of the Amphibia. On 

 those of fishes these motar organisms do not 

 exist. Wherefore this distinction ? Why 

 should they exist on the external and not on 

 the internal gills? It is not a law of the 

 mucous membrane that they should not exist, 

 for they occur in other tracts of this same 

 structure. These are questions of ultimate 

 design which it is not given to science to 

 answer. 



Air-bladder of fishes. This organ repre- 

 sents the prototypal form of " the lung" in 

 the animal kingdom. It is present in nearly 

 all osseous fishes. It is always tensely filled 

 with gas. In that of marine fishes, oxygen 

 predominates ; in that of fresh water, nitrogen. 

 Humboldt found the gas in the air-bladder 

 of the electric gymnotusto consist of 96 parts 

 of nitrogen, and 4 of oxygen. Biot found 87 

 parts of oxygen, nitrogen, and carbonic acid in 



the deep Mediterranean fishes. No hydrogen 

 has ever been detected in this organ. It 

 occupies the roof of the abdomen, between 

 the kidneys and chylopoaetic viscera, and 

 sometimes (gi/innottts ophiocephalus, coins), 

 beneath the caudal vertebra to nearly the 

 end of the tail. In some species of diodtni, 

 tetradon, dactylapterus, pemelodits, and poia- 

 notus, it is bifurcated. In arius gagora, poly- 

 ptcrus and lepidosiren, it is divided lengthwise 

 into two bladders. In the cyprinidce and 

 characinidic it is divided transversly into two 

 communicating compartments. Many other 

 varieties of form occur. (Vide art. PISCES.) 

 The proper walls of the air-bladder consist of 

 a shining silvery fibrous tunic, the fibres being 

 arranged for the most part transversely and 

 circularly, and in two layers. They are con- 

 tractile and elastic. This coat yields the 

 finest gelatine. Its fibres belong to the white 

 variety: they "swell" under the action of 

 acetic acid. A stratum of vessels is inter- 

 posed between the mucous membrane and 

 the fibrous layer. The meshes formed by 

 these vessels are considerably larger and more 

 oblong than those of the pulmonary capil- 

 laries. In the latter instance the meshes 

 exceed the vessels in diameter. The arteries 

 of this organ are derived sometimes from the 

 abdominal aorta, sometimes from the caeliac 

 arterv, sometimes from the last branchial vein ; 

 and in the lepidosiren they are continued from 

 the aortic termination of the two non-rami- 

 fied branchial arteries, and therefore convey 

 venous blood to the cellular, lung-like, double 

 air-bladder (Owen). The veins of the air- 

 bladder return, in some fishes, to the portal 

 vein ; in some to the hepatic vein ; in some 

 to the great cardinal vein ; and in the lepido- 

 siren, they penetrate by a common trunk the 

 great portal vein formed by the confluence of 

 the visceral and vertebral veins of the trunk. 

 In the protopherus and ganoid fishes the 

 vessels of this organ form no retia mirabilia 

 and vaso-ganglions, but rather a diffused 

 capillary network, more close and rich in the 

 anterior than the posterior part. In the 

 osseous fishes, several varieties of the vascular 

 system of this organ occur. That of the carp 

 forms tufts of capillaries throughout the 

 whole interior of the organ, a variety of 

 which tufts occurs in the pike. The perch 

 and cod exhibit a vaso-ganglion, a body pe- 

 culiar to the air-bladder of fishes. In the 

 cod-fish, a large artery, a branch of the cceliac, 

 and a still larger vein, which empties itself 

 into the mesenteric, perforate together the 

 fibrous tunic of the bladder. Before they 

 reach the inner surface, they divide into 

 some branches which then radiate and sub- 

 divide upon the mucous membrane. The 

 arterioles frequently anastomose with each 

 other. Both are inextricably interwoven, and 

 form the basis of the so-called " air-gland," 

 which is essentially a larger "bipolar rete 

 mirabile" (Miiller), or vaso-ganglion. In the 

 cod the ultimate vessels of this gland have a 

 loop-like arrangement, their free surface (a ), 

 being covered over with another layer of 



