ALIMENTARY TRACT OF THE KING SALMON 
97 
MUSCULAR COATS. 
The muscular coats are the inner circular and the outer longitudinal muscles. The 
two coats are not so thick as in the intestinal wall or in the thick muscular walls of the 
stomach. But they are on the whole very regular and uniform. 
Muscularis circularis . — The circularis is thicker than the longitudinalis in the 
salmon csecum in the approximate ratio of 3 to 2. The coat is composed of relatively 
long, smooth muscle fibers. When the caecum is distended the number of fibers present 
in the total diameter of this coat in a transverse section is only from 8 to 10, often less. 
Numerous capillaries are always present. Very rarely in the normal animal will one 
find a granule cell in the spaces of the muscle fibers of the circular muscle coat. 
Just at the base of the caecum the muscle is a little thicker and the lumen usually 
smaller. The structure shows all the appearances suggesting a degree of valvelike 
action capable of influencing the filling or emptying of the caecum from or into the cavity 
of the pyloric intestine out of which it originates. 
Muscularis longitudinalis . — The longitudinalis is a thin sheet of smooth muscle, the 
outermost of the muscle coats. In the king salmon the longitudinal coat varies in 
thickness according to the distension of the tube, but on an average it is from 5 to 8 
fibers thick. 
This coat is chiefly distinguished by its vascularity. Relatively large veins are 
found at intervals in the coat and occasionally a section will show arterioles entering 
the caecum from the exterior, penetrating this coat to distribute branches out in the 
space between the two muscle coats. In the neighborhood of these blood vessels there 
is connective tissue in relatively large amounts, an arrangement which tends to break 
up the continuity of the muscular sheet. 
On the inner surface of the longitudinal muscle coat between it and the circular 
muscle fibers, sections reveal the presence of numerous nerve cells and fibers of the 
plexus of Auerbach. In the individual ganglia of this plexus there are only a few nerve 
cells. 
SEROUS COAT. 
The serous coat of the cylindrical cseca. of the king salmon consists of a single layer 
of squamous epithelial cells resting on a subserous connective tissue base. In the 
normal tissue these cells are extremely thin and attenuated. Their boundaries are 
difficult to measure in transverse section, but the extent is from 10 to 12 g. The nuclei 
are spindle shaped, having a diameter of about i /r and a length of about 5 g. In many 
specimens the serous cells are more cubical in outline and the coat is correspondingly 
thicker. 
