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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 

The Development of the Digestive Tube.—The primitive digestive tube consists of two parts, viz.: (1) the fore-gut, within the cephalic flexure, and dorsal to the heart; and (2) the hind-gut, within the caudal flexure (Fig. 977). Between these is the wide opening of the yolk-sac, which is gradually narrowed and reduced to a small foramen leading into the vitelline duct. At first the fore-gut and hind-gut end blindly. The anterior end of the fore-gut is separated from the stomodeum by the buccopharyngeal membrane (Fig. 977); the hind-gut ends in the cloaca, which is closed by the cloacal membrane.


FIG. 977– Human embryo about fifteen days old. Brain and heart represented from right side. Digestive tube and yolk sac in median section. (After His.) (See enlarged image)


The Mouth.—The mouth is developed partly from the stomodeum, and partly from the floor of the anterior portion of the fore-gut. By the growth of the head end of the embryo, and the formation of the cephalic flexure, the pericardial area and the buccopharyngeal membrane come to lie on the ventral surface of the embryo. With the further expansion of the brain, and the forward bulging of the pericardium, the buccopharyngeal membrane is depressed between these two prominences. This depression constitutes the stomodeum (Fig. 977). It is lined by ectoderm, and is separated from the anterior end of the fore-gut by the buccopharyngeal membrane. This membrane is devoid of mesoderm, being formed by the apposition of the stomodeal ectoderm with the fore-gut entoderm; at the end of the third week it disappears, and thus a communication is established between the mouth and the future pharynx. No trace of the membrane is found in the adult; and the communication just mentioned must not be confused with the permanent isthmus faucium. The lips, teeth, and gums are formed from the walls of the stomodeum, but the tongue is developed in the floor of the pharynx.
  The visceral arches extend in a ventral direction between the stomodeum and the pericardium; and with the completion of the mandibular arch and the formation of the maxillary processes, the mouth assumes the appearance of a pentagonal orifice. The orifice is bounded in front by the fronto-nasal process, behind by the

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