242 CHORDATE ANATOMY 



The liver diverticulum projects into the ventral mesentery and the meso- 

 derm of the septum transversum which separates the pericardial cavity 

 from the abdominal cavity. The outgrowth soon becomes differentiated 

 into an anterior mass of branching cords surrounded by branches of the 

 vitelline veins, and a posterior hollow sac which later becomes the gall 

 bladder. The multiphcation of the cords, correlated with that of the 

 blood capillaries associated with them, produces the lobules. Mesenchyme 

 cells form the interlobular connective tissue. Bile capillaries appear 

 within the cell cords, which thus become hepatic tubules, and the blood 

 capillaries acquire endothelial walls. As a result of this, the lumen of each 

 bile capillary is separated from that of each blood capillary by a layer of 

 gland cells and a layer of endothelial cells. (Figs. 217, 221, 226) 



The multiplication of tubular cords and of blood spaces results in a 

 rapid enlargement of the liver, which begins to bulge out from the septum 

 transversum and the ventral mesentery and to push into the abdominal 

 cavity between the septum and the stomach. In this way the liver 

 becomes covered by the peritoneum. Meanwhile it acquires its two 

 chief lobes. The ventral mesentery into which it originally grew forms 

 the falciform ligament. (Fig. 221) 



History of the Liver. The vertebrate liver has no homolog among 

 invertebrates, though many of these have organs which are called livers. 

 The liver of Amphioxus is generally regarded as representing the beginning 

 of that of vertebrates. This is a ventral outpocketing of the intestine 

 immediately behind the pharynx. It grows ventrally and forwards 

 beneath the pharynx, and remains a hollow sac throughout life. Its rela- 

 tions to the blood-vessels resemble those of the liver of vertebrates. 



The liver becomes bilobed in cyclostomes and elasmobranchs, and a 

 gall bladder is differentiated. In the higher vertebrates and man no 

 important morphological changes occur. The form, however, varies 

 with the shape of the abdominal cavity and the pressure of surrounding 

 organs. 



The Pancreas 



The pancreas is a light pinkish organ about five inches in length, 

 extending across the abdominal cavity from a loop of the duodenum on 

 the right side to the left colic flexure. In man the pancreas usually has 

 two functional ducts. One of these, the pancreatic or Wirsung's duct, 

 generally opens into the common bile duct; the other, the accessory or 

 Santorini's duct, opens into the duodenum about an inch above the opening 

 of the bile duct. 



The pancreas secretes trypsinogen, which is converted into trypsin 

 through the action of enterokinase secreted by the intestinal glands. 

 Tryj)sin splits proteins into amino-acids. The enzyme amylopsin secreted 



