. The chordates. Chordata. Spiracle Heart Pectoral girdle Mouth-cavity Pharynx Esophagus vertebral column Neural arch. Mouth-cavity Pharynx Esophagus Fig. 18. Divisions of digestive tube. (A) Shark. (B) Beptile. Begions sectioned are cut in sagittal plane, except the nasal sac which, being one of a pair of lateral organs, is shown in parasagittal section. Throughout the greater portion of the length of the digestive tube, the muscular part of its wall consists of an external layer of longitudi- nal muscle fibers and an internal layer of circular fibers encircling the tube (Fig. 20). The fibers
. The chordates. Chordata. Spiracle Heart Pectoral girdle Mouth-cavity Pharynx Esophagus vertebral column Neural arch. Mouth-cavity Pharynx Esophagus Fig. 18. Divisions of digestive tube. (A) Shark. (B) Beptile. Begions sectioned are cut in sagittal plane, except the nasal sac which, being one of a pair of lateral organs, is shown in parasagittal section. Throughout the greater portion of the length of the digestive tube, the muscular part of its wall consists of an external layer of longitudi- nal muscle fibers and an internal layer of circular fibers encircling the tube (Fig. 20). The fibers are ordinarily of the simpler "smooth" or nonstriated sort, but in the more anterior part of the esophagus the layers may consist of the more complex striated fibers (see p. 312). Sphincters, such as the pylorus, are thickened rings of the circular layer. The extent of internal surface available for the secretory and ab- sorptive activities of the tube is increased in various ways. The ab- dominal part of the tube is usually much elongated, even to the extent. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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