Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29464/1066/
982 
PISCES. 
tube passing directly from the stomach to the 
anus. In the Mullets ( Mugil), on the con¬ 
trary, the intestine is of great length and is very 
curiously folded, being intervolved with the 
folds of the liver so as to form a mass of trian¬ 
gular form moulded to the shape of the abdo¬ 
minal cavity, and affording an example of the 
longest and, in its disposition, the most complex 
intestinal canal of any of the class. 
This arrangement, as Rathke very justly ob¬ 
serves, is an obvious approximation to the dis¬ 
position of the viscera most generally met with 
in molluscous animals, in which the folds of 
the intestine are prolonged into and almost bu¬ 
ried among the folds of their very large liver. 
In the Electric Eel ( Gymnotus electricus) 
the disposition and termination of the intestinal 
tube is curious. First descending to the lower 
end of the stomach, it passes to the left side 
and ascends again as far as the oesophagus ; it 
then winds downwards and backwards so as to 
encircle the stomach, and, lastly, advancing for¬ 
wards along the ventral aspect of the abdomen, 
it terminates, as in the Cephalopoda, beneath 
the throat, in the immediate vicinity of the 
heart and root of the tongue. 
No fish has anything like a colon or cæcum. 
The only distinction between small and large 
intestines is met with just at the termination of 
the alimentary tube, where it opens into a kind 
of cloacal cavity, usually called the rectum. At 
this point there is generally a prominent cir¬ 
cular fold of the lining membrane constituting 
a kind of valve. In the Salmon several of these 
valvular zones succeed each other, giving to this 
part of the gut an appearance similar to that of 
the intestine of the Plagiostome cartilaginous 
Fishes immediately to be described. 
In the Sharks and Rays, the Plagiostomes 
Fig. 519. 
Alimentary canal of Shark, 
a, oesophagus ; b, b, cavity of stomach, at the 
commencement of which are placed the valvular 
fringes mentioned in the text ; c, passage leading to 
pylorus ; d, spleen ; e, pyloric cavity ; f, dilated 
chamber; g, h, bile-ducts; », orifice of pancreatic 
duct ; h, k, valvular intestine. 
just referred to, and also in the Sturgeon, the 
intestine presents a very remarkable structure. 
Externally it resembles a wide bag nearly simi¬ 
lar in shape and size to the stomach itself, and 
so short and stunted that, without some special ■ 
arrangement, obviously a sufficient surface ; 
would not be afforded for the absorption of the 
nutritious portions of the food. By the me- ; 
chanism adopted, however, this is abundantly 
provided. Throughout the whole length of the ■ 
gut the mucous membrane is arranged in deep 
spiral folds (fig. 519), which wind from end to 
end, only leaving a small orifice in the centre of 
each valvular projection, whereby the different 
compartments formed between the spiral lamina 
can communicate with each other, so that the j 
digested food by this unusual arrangement is * 
spread over a very great superficial area, and all 
the benefits of a long and convoluted intestinal 
tube are secured. Each fold of this extensive 
spiral valve contains between the layers of mu- j 
cous membrane that compose it an elastic 
substance, whereby it is kept constantly spread 
out and restored to its original position when 
displaced by the passage of food through the 
central channel that permeates the whole series. ( 
In the Sturgeons a similar valve exists, but 
its spiral folds are not so closely arranged : the 
intestine, moreover, is remarkable on account 
of the great thickness of its muscular and in- f 
ternal tunics, the latter of which presents a 
reticulated or honey-combed appearance, the 
larger meshes including irregular spaces, which j 
are again subdivided into smaller cells. Slight j 
vestiges of the spiral intestinal valve are visible 
even in the Lamprey. 
Salivary glands.—From the circumstances ] 
under which Fishes swallow their food, the 
presence of any salivary apparatus is evidently j 
uncalled for; no fish, therefore, possesses true 
salivary glands. Nevertheless, in the Cypri- 
nidæ and some other races the whole palate is i 
covered with a soft spongy substance, from 
which a kind of mucosity is discharged through ; 
imperceptible pores, which has been regarded . 
by Rathke and others as a salivary organ : 
Cuvier, however, denies the glandular character 
of this substance, regarding it as a peculiar and I 
highly sensible tissue destined to be the seat of j 
a sense more or less analogous to that of taste, 
—a supposition that is rendered more probable I 
from the great number of nerves that enter its j 
substance. 
Pancreas.—In the osseous Fishes no pan- I 
créas, such as that met with in the higher I 
r classes of vertebrate animals, exists ; it is, 1 
.^however, represented by a variable number of I 
ccecal appendages, which open into the duode- I 
num in the vicinity of the pylorus. The lining I 
membrane of these pyloric cceca is of a glan-1 
dular character, and secretes an abundance of | 
a thin glairy fluid analogous to the pancreatic I 
secretion. The existence of the appendages in I 
question is, however, by no means constant;! 
thus in the Labrida, the Siluridæ, the Cypri- ! 
nidre, and many members of the pike genus, j 
they are altogether wanting. When present,! 
moreover, their number varies very remarkably! 
in different Fishes; thus, sometimes, as in thé!
        

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