. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. ALIMENTARY CANAL OF FISHES. 423 283 SpnilvalM, Zi/t/fnia. 284 and Z^r/cena.^ In the second and more common modification, the fold of mucous membrane is disposed in close transverse coils, as shown in the longitudinal section of the Selachc's gut, fig. 284, h ; and a transverse section exposes only tlie fiat surfirce of one of the coils. In the Fox-shark (Alopias I ulpes) ; the valve describes thirty-four circum- gyrations within seven inches' extent of the intestine ; the mucous membrane is minutely honeycombed: a few sc
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. ALIMENTARY CANAL OF FISHES. 423 283 SpnilvalM, Zi/t/fnia. 284 and Z^r/cena.^ In the second and more common modification, the fold of mucous membrane is disposed in close transverse coils, as shown in the longitudinal section of the Selachc's gut, fig. 284, h ; and a transverse section exposes only tlie fiat surfirce of one of the coils. In the Fox-shark (Alopias I ulpes) ; the valve describes thirty-four circum- gyrations within seven inches' extent of the intestine ; the mucous membrane is minutely honeycombed: a few scattered fibres of elastic or involuntary muscular tissue may be traced in the vasculo-cellular layer included within the mucous fold, and they form a slender band within the free border of the valve, retaining much elasticity in the dead intestine, and drawing that border into festoons. Besides Seladie, fio;. 284, li, and Alopias, the spiral valve is transverse in Galeus, Lamna, and all the Dog-fishes {^Scylliidce and SpinacidfE). The trunk of the ' arteria meseraica intestinalis,' and that of the corresponding veins of the longi- tudinally convoluted valve, run along its free thickened border, and the vein quits its commencement to join the vena porta;: ^ the arterial and venous trunks of the trans- versely spiral valve are external to the gut. One may connect the peculiarity of the spiral valve with the necessity for reducing the mass and weight of the abdominal contents in the active high-swimming Sharks, which have no swim-bladder: the essential part of an intestine being its secern- ing and absorbing surface, we see in them the requisite extent of the vasculo-mucous membrane packed in the smallest comp)ass, and associated with the least possible (tuantity of accessory muscular and serous tunics, by the modifications above described. Analogous ones exist, however, in other Plagiostomes, and in the Lamprey, to which the above physiological exijlanation will not ajiply; and t
Size: 1393px × 1794px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorowenrichard18041892, bookcentury1800, bookdecade1860
