FISHERY BULLETIN: VOL. 71. NO. 2 



Figure 10. — The esophageal-intestinal 

 valve of a 240-mm SL cunner ( X 40). 

 E, esophagus: I, intestine; mf, mucosal 

 fold; sm, smooth muscle; Sm, sub- 

 mucosa. 







f 



mf 





40^>-^ 



^^•S^.' 





^*^sac:^ 







\ 



^''^, 



sm 





-J/?-*^ 

 •,•.->« 





^ ■-■^.':j-li^ 



MUSCULARIS AND SEROSA.— A very 

 thick circular layer of striated muscles con- 

 tinues from the pharynx forming the external 

 muscular layer of the esophagus (Figures 7, 8). 

 It is invaded by elements of the vascular system 

 accompanied by connective tissue; thus it is a 

 loose rather than a compact layer. This mus- 

 cularis appears to decrease in thickness pos- 

 teriorly toward the esophageal-intestinal valve, 

 where it forms a triangular, sphincterlike struc- 

 ture along the base of the valve fold. The basal 

 muscularis of the valve is formed of smooth 

 and striated muscles and retains significant 

 connective tissue components. Leucocytes, fibro- 

 blasts, and a few granular cells also occur in 

 this connective tissue. The distal portion of the 

 valve contains a single undivided smooth muscle 

 layer (Figure 10). This ring is continuous with 

 the circular muscle layer of intestine. 



The anterior esophagus is attached to the 

 associated skeletal muscles by a layer of ad- 

 ventitious tissue. After passing through the 

 transverse septum, the esophagus separates 

 from the visceral wall ventrally. The dorsal 

 wall of the midesophagus is attached to the 

 kidney and skeletal muscles by a thin layer 

 of adventitia (Figures 7, 8). The ventral side is 

 covered by the peritoneal lining (serosa) and 

 is separated from the circular muscularis by a 

 varying thickness of areolar connective tissue 

 (Figure 7). Posteriorly, the serosa increases in 



thickness in the vicinity of the esophageal- 

 intestinal valve. 



Intestine 



The intestine starts posterior to the esopha- 

 geal-intestinal valve and extends to the rectal 

 valve. The intestine can be divided into four 

 sections (Figure 1). The first section (I) begins 

 dorsally and proceeds posteriorly and slightly 

 to the left. It turns ventrally and extends 

 anteriorly to the right as section II. The second 

 loop curves abruptly posterior at about the 

 level of the intestinal bulb. The third section 

 (III) proceeds posteroventrally to an S-shaped 

 loop which is connected by a short, straight 

 portion of the intestine to the rectum. Intestinal 

 veins from sections I, II, III, and the S-loop 

 are associated with the anterior mesenteric 

 vein and the posterior mesenteric vein from 

 the rectum (Figure 1). The intestine has a 

 consistent histological organization throughout 

 its length with minor cytological variations. 

 The mucosa is more complex in the intestine 

 than in the esophagus and has zigzag folds as 

 well as secondary and tertiary folds. The in- 

 testinal bulb is a dilation of the anterior por- 

 tion of the intestine where the latter joins the 

 esophagus. The mucosal folds of the intestinal 

 bulb are deeper than in other portions of the 

 intestine. There are four distinguishable layers 



572 



