THE ABDOMEN 



799 



^- Lung 



CEsophagus 



Stomach 



Bile duct 



it opens into the mid-gut. The part of the mid-gut which is connected with 

 this duct gives rise to the lower portion of the ileum. The duct, as a rule, 

 becomes closed and disappears. In some cases, however, its intestinal portion 

 retains its communication with the gut, and gives rise to a bhnd evagination 

 of the gut, which is known as Meckel's diverticulum. In very rare cases the 

 intestinal portion of the vitelline duct undergoes no closure, under which cir- 

 cumstances it is pervious throughout, opening internally into the gut, and 

 externally into the cavity of the umbilical cord. Such a condition of matters 

 gives rise to a congenital umbilical feecal fistula. 



The primitive gut is lined throughout with entoderm, from which the future 

 intestinal epithelium is developed, as well as the epithelium of the gland-ducts 

 which open into the gut. This entoderm is covered externally by splanchnic 

 ■mesoderm, these t^vo combined layers forming the splanchnopleure. The 

 splanchnic mesoderm gives origin to the tissues which form the other coats of 

 the gut. 



The primitive gut, thus constituted, is separated from the body-wall, which 

 is composed of somatopleure (ectoderm 

 and somatic mesoderm combined) by the 

 coelom or body-cavity, which becomes 

 partitioned ofE into pericardial, pleural, 

 and peritoneal ca\'ities. 



Stomach. — The primitive gut is a 

 straight tube up to about the fourth week 

 of intra-uterine hfe. About that time the 

 first indication of the stomach shows 

 itself as a spindle-shaped dilatation of the 

 primitive gut in the neighbourhood of 

 the embryonic heart, this dilatation 

 being at first straight. After the stomach 

 descends into the abdominal cavity its 

 dorsal aspect undergoes greater develop- 

 ment than the ventral, and the organ 

 becomes curved. The convexity of this 

 curve is, at this period, directed dorsal- 

 wards, and the concavity ventralwards. 

 The surfaces of the organ are right and 

 left, and they thus naturally receive, at 

 this period, the corresponding pneumo- 

 gastric nerves. The pyloric end, which is 

 directed caudalwards, is now tilted for- FiG. 344. — The Alimentary 

 wards towards the ventral body-wall, and Canal of the Embryo (His). 

 it carries the duodenal portion of the gut 



along with it, the duodenal loop being thereby produced. Thereafter the 

 stomach undergoes parried rotation round its long axis towards the right side, 

 and the cephalic or cardiac end is carried to the left of the median line. The 

 stomach, as it rotates, carries with it the lower part of the oesophagus and the 

 duodenal loop, which latter thus takes up a position on the right side of the 

 median hne. 



The original right surface of the stomach now becomes dorsal or posterior, 

 and the original left surface becomes ventral or anterior. This change in the 

 direction of the surfaces explains why the right pneumogastric nerve is dis- 

 tributed to the posterior surface of the stomach in the adult, and the left pneumo- 

 gastric nerve to the anterior surface of the organ. The partial rotation of the 

 lower part of the oesophagus towards the right also explains why, in the adult, 

 the right pneumogastric nerve lies behind the lower part of the oesophagus, and the 

 left in front of the lower part of that tube. 



The great curvature of the stomach, which was originally directed dorsal- 

 wards, is now directed caudalwards and towards the h ft ; whilst the small curva- 

 ture, which originally looked ventralwards, is now directed cephalicwards or 

 upwards, and towards the right. 



