2l8 



HISTOLOGY. 



dance of veins in the submucosa of the anal part of the rectum should be 

 noted because of its clinical importance. 



LIVER. 



The liver is one of the three organic glands which develop from the 

 digestive tube, the others being the pancreas and the lungs. 



Development of the liver. The liver arises as a clump of rounded 

 masses of entodermal cells which proliferate from the ventral surface of the 

 "pharynx" just anterior to the yolk sac. It is shown in the diagram Fig. 

 245, A. The liver at this stage lies between the vitelline veins, in the con- 

 nective tissue which extends from the mesothelium of the pericardial cavity 

 to the entodermal layer of the yolk sac. Since this connective tissue forms 



p.c. 



y.s 



FIG. 245. DIAGRAMS OF THE DEVELOPMENT OF THE LIVER. 



A, The condition in a 4.0 mm. human embryo. B. A 12 mm. pig. C, The arrangement of ducts in the 

 human adult, c. d., Cystic duct; c. p., cavity of the peritonaeum; d., duodenum; d. c., ductus chole- 

 dochus; dia., diaphragm; div., diverticulum; f. 1., falciform ligament; g. b., gall bladder; g. o., 

 greater omentum; h. d., hepatic duct; ht., heart; int., intestine; li., liver; 1. o., lesser omentum; m., 

 mediastinum; oe., oesophagus; p. c., pericardial cavity; p. d., pancreatic duct; ph., pharynx; p. v., 

 portal vein; St., stomach; tr., trabecula; v. c. i., vena cava inferior; v.v., vitelline vein; y. s., yolk sac. 



a septum across the body, separating the cavity of the yolk sac from that 

 of the pericardium, it is called the septum transversum. With further 

 growth the liver becomes divisible into two parts; first, a more or less cylin- 

 drical diverticulum of the intestine (Fig. 245, B, div.} ; and second, a mass 

 of "branched columns of entodermal cells, the hepatic trabeculae, which 

 grow out from the diverticulum and form the essential part of the liver 

 (Fig. 245, B, tr.}. The trabeculae are not irregular detached islands as seen 

 in single sections, but through anastomosis with one another they form a 

 single complex network of solid cellular cords. At first they are connected 

 with the diverticulum by several strands of cells, as in B, but later all of 

 these atrophy and disappear except one, which forms a permanent com- 

 munication between the trabeculae and the diverticulum. After acquiring 



