886 



FORMATION OF THE INTESTINAL CANAL. 



liver to reach the heart. They send branches carrying arterial blood to the liver, and the 

 latter grows round these vessels. These branches are the venue advehentes (2 and 3, a). The 

 blood circulating through the liver from the venre advehentes is returned by other veins, the 

 vewe revehentes (2 and 3, r), which reunite at the blunt margin of the liver with the chief 

 trunk of the umbilical vein. The umbilical vein (3, w x ) and the omphalomesenteric vein 

 (3 oi,) anastomose in the liver. When the intestine develops (3, D), the mesenteric 

 vein (mi! oi>ens into the omphalomesenteric vein, and the splenic vein as well (4, I), 

 when the spleen is formed. At a later period, when the omphalo-mesenteric vein (4, om x ) 

 disappears, the vein from the intestine now becomes the common trunk of the previously 

 united vessels. It unites in the liver with the umbilical vein to form the trunk of the vena 

 i>ort;-. When, after birth, the umbilical vein disappears (4, u x ), the mesenteric alone remains 

 as the portal vein. As the ductus venosus is obliterated, the portal vein must send its blood 

 through the liver, and thus the portal circulation is completed. 



449. FORMATION OF THE INTESTINAL CANAL. The primitive intestine, or gut, 

 consists'of a straight tube proceeding from the head to the tail. The vitelline duct is inserted 

 at that point, which at a later period corresponds to the lower part of the ileum. At the 4th 

 week the tube makes a slight bend toward tne umbilicus (fig. 679, I). As already mentioned, 



the vitelline duct 

 is obliterated, re- 

 maining only for a 

 time as a thread 

 attached to the in- 

 testine, being still 

 visible at the 3rd 

 month. Sometimes 

 it re mains as a short 

 blind tube com- 

 municating with 

 the intestine. This 

 is the so -called 

 ' ' true intestinal 

 diverticulum"; oc- 

 casionally a cord 

 Fig. 680. the obliterated 



stomach; o, insertion of the omphalo-mesente- 

 ric vessels passes 

 from it to the um- 

 bilicus. In very 

 rare cases, the duct 

 may remain open 

 as far as the um- 

 bilicus, forming a congenital fistula of the ileum, or it may give rise to cystic formations (M. 

 Jioth). In a human foetus at the 4th week, His distinguished the cavity of the mouth, pharynx, 

 tesophagus, stomach, duodenum, mesenterial intestine, and the hind-gut, with the cloaca. The 

 intestine then forms the first coil (fig. 679, II) by rotating on itself at the intestinal umbilicus, 

 so that the lower part of the intestine lying next the knee-like bend comes to lie above, while 

 the upper j>art lies below. From the lower part of this loop, there proceed the coils of the 

 small intestine (III, t), which gradually grow longer. From the upper limb of the loop, 

 which also elongates, the large intestine is formed ; first the descending colon, then by elonga- 

 tion the transverse colon, and lastly the ascending colon. 



Glands. By diverticula, or protrusions from the intestine, the various glands are formed. 

 The cells of the hypoblast proliferate and take part in the process, as they form the secretorv 

 cells of the glands, while the mesoblastic part of the splanchnopleure forms the membranes o"f 

 the glands, giving them their shai>e. The diverticula are as follows : 



1. The salivary glands, which grow out from the oral cavity at first as simple solid buds, 

 but afterwards become hollow and branched. [The salivary glands are developed from the 

 epiblast lining the mouth (stomodwum).] 



2. The lungs, which arise as two separate hollow buds (fig. 680, A, 2), and ultimately have 

 only one common duct, are protrusions from the (esophagus. The upper part of the united 

 tracheal tube foi-ms the larynx. The epiglottis and the thyroid cartilage originate from the 

 part which forms the tongue (Ganghofner). The two hollow spheres grow and ramify like 

 branched tubular glands with hollow processes (B, /). In the first period of development, there 

 is no essential difference between the epithelium of the bronchi and that of the primitive air- 

 vesicles {iUusda). The spleen and suprarenal capsules, however, are not developed in this way. 

 J he former anses in a fold of the mesogastrium at the 2nd month {His) ; the latter are origin 

 ally larger than the kidneys. 



v, 



Fig. 679. 

 Fig. 679. Development of the intestine, 

 vitelline duct ; t, small intestine ; c, colon ; r, rectum. Fig. 680. For- 

 mation of the lungs. A, Diverticula of the lungs as double sacs k, 

 mesoblastic layer ; I, hypoblastic layer ; m, stomach ; s, oesophagus. B, 

 Further branching of the lungs t, trachea ; b, e, bronchi ; /, projecting 

 vesicles. 



