THE DIGESTIVE SYSTEM 
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covers the whole outer surface forming the very thin serous coat 
(tunica serosa); within this, ( b ) the thick, firm muscular coat 
(tunica muscularis) consisting of a thin outer layer of longitudinal 
fibers which may be easily stripped off (together with the serosa), 
and a thicker inner layer of circular fibers; inside the muscularis 
(c) a layer of connective tissue (tela submucosa); and finally (d) 
the innermost layer (tunica mucosa) which is very conspicuous 
because of the innumerable absorbent organs (villi) which thickly 
cover its surface and project into the lumen. Note the velvety 
appearance which the inner surface presents when a short length 
of intestine is laid open and washed. 
Microscopic Study (Lab. SI. Coll.).—Examine cross sections 
of the intestine of an amphibian and identify the various coats 
above mentioned. Note that the serosa is continuous with the 
mesentery and consists of a single layer of flat cells showing in 
cross section as a thin line with occasional flattened nuclei; that 
the spindle shaped involuntary muscle fibers (each a single cell 
with a single nucleus) of the longitudinal layer are cut transversely 
and those of the circular layer are cut longitudinally; that there 
are numerous blood vessels in the various layers, particularly 
in the submucosa; that the mucosa has no villi as has the mam¬ 
malian intestine but is thrown into numerous longitudinal folds, 
here cut transversely, which would disappear if the intestine 
were distended with food; that these folds are covered by a simple 
layer of columnar epithelial cells among which are many cells 
of the goblet type, i.e., cells which pour out their mucous secre¬ 
tion from time to time into the intestine; that there are a few 
multicellular intestinal glands of the coiled tubular type, lined 
with simple epithelium and opening into the intestinal lumen 
between the folds. Draw the whole section on as large a scale as 
the size of the page will allow , and show the outlines of the various 
layers, filling in, in their proper places, representative details of 
cell structure. 
Examine cross sections of the small intestine of the rabbit, for 
general identification of layers. Note that here the muscular 
layers are very thin; that the mucosa possesses in addition to a few 
large folds, numerous villi, and a much larger number of intestinal 
glands, which are also larger and more complicated. 
