. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Intake Apparatus 271 only one cell-layer thick, except in the anterior esophageal region. It per- forms not only the "root hair" function of absorption, but also gives rise to various digestive glands whose secretions bring about chemical transforma- tion of the food taken in. All the other layers aside from the mucosa are secondary and are subsequently added to this most important absorptive primary lining of the food tube. The submucosa, next t
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Intake Apparatus 271 only one cell-layer thick, except in the anterior esophageal region. It per- forms not only the "root hair" function of absorption, but also gives rise to various digestive glands whose secretions bring about chemical transforma- tion of the food taken in. All the other layers aside from the mucosa are secondary and are subsequently added to this most important absorptive primary lining of the food tube. The submucosa, next to the mucosa, is largely devoted to supporting a rich network of capillaries and lymphatics which bear away over the body the materials absorbed by the mucosa. Blood vessels _ .£, ,. , ,, Epithelial cell Goblet cell. Lumen of intestine Serosa (peritoneum) Mucosa Submucosa Circular muscles Longitudinal muscles Fig. 219. Part of a cross section through a frog's intestine. (From Mavor, General Biology, copyright 1947, by permission of The Mac- millan Company, publishers. After Holmes.) Outside of the submucosa there is a double layer, the muscularis, com- posed of circular muscles on the inside and longitudinal muscles on the outside. These muscles are involuntary in their action, except for a short distance at either end of the tube in the stomodaeal and proctodaeal re- gions, where they are under the control of the will. They effect movement of the food through alternate contractions by processes of segmentation and peristalsis. Segmentation churns the contents of the tube back and forth, while peristalsis forwards it. Protecting the muscular layers on the outside is a sustentative layer of tissue called the serosa, which is continuous with the mesenteries and with the peritoneum that lines the body cavity. In that part of the tube lying outside of the abdominal cavity, no serosa is present. 5. Regions of the Tube Since food undergoes progressive modification as it passes throu
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