The development of the human body; a manual of human embryology . ated with these structures persist, however, and giverise to the muscles of the pharynx, which consequently areinnervated by the ninth and tenth nerves. The Development of the (Esophagus.—From the ventralside of the lower portion of the pharynx an evaginationdevelops at an early stage which is destined to give rise tothe organs of respiration; the development of this may,however, be conveniently postponed to a later chapter(Chap. XII). The oesophagus is at first a very short portion of thearchenteron (Fig. 168, A), but as the he
The development of the human body; a manual of human embryology . ated with these structures persist, however, and giverise to the muscles of the pharynx, which consequently areinnervated by the ninth and tenth nerves. The Development of the (Esophagus.—From the ventralside of the lower portion of the pharynx an evaginationdevelops at an early stage which is destined to give rise tothe organs of respiration; the development of this may,however, be conveniently postponed to a later chapter(Chap. XII). The oesophagus is at first a very short portion of thearchenteron (Fig. 168, A), but as the heart and diaphragmrecede into the thorax it elongates (Fig. 168, B) until iteventually forms a considerable portion of the digestivetract. Its endodermal lining, like that of the rest of the 3i8 THE DEVELOPMENT OF THE HUMAN BODY. digestive tract except the pharynx, is surrounded bysplanchnic mesoderm whose cells become converted intonon-striated muscular tissue, which by the fourth monthhas separated into an inner circular and an outer longitudi-nal layer. Rj,. Fig. 168.—Reconstructions of the Digestive Tract of Embryos of (a) mm. and (b) 5 mm. all, Allantois; cl, cloaca; /, lung; li, liver; Rp, Rathkes pouch; S, stomach; /, tongue; th, thyreoid body; Wd, Wolffian duct; y, yolk-stalk.—(His.) The Development of the Stomach and Intestines.—By the time the embryo has reached a length of about 3 constriction from the yolk-sac has proceeded so far thata portion of the digestive tract anterior to the yolk-saccan be recognized as the stomach and a portion posterioras the intestine. At first the stomach is a simple spindle- THE STOMACH. 319 shaped enlargement (Fig. 168) and the intestine a tubewithout any coils or bends, but since in later stages theintestine grows much more rapidly in length than the ab-dominal cavity, a coiling of the intestine becomes neces-sary. The elongation of the stomach early produces changesin its position, its lower end bending over toward
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
