124 BIOLOGY OF THE LABORATORY MOUSE 



rectum is similar structurally to the colon. There is a gradual increase in 

 the thickness of the inner circular smooth muscle layer of the muscularis 

 externa. The serous covering is replaced by the loose connective tissue of 

 the adventitia, which attaches the rectum to the surrounding tissue. 

 Toward the anal opening the glands become shorter and disappear as the 

 lining of the lumen changes into thick, stratified squamous epithelium which 

 is continuous with the thin, stratified squamous epithelium of the skin. 

 The musculature at the anus is composed of striated muscle fibers. Sur- 

 rounding the anus are the anal glands. They are similar structurally to 

 sebaceous glands and are arranged in small lobules separated by con- 

 nective tissue. 



Mesenteries. — The stomach and the intestines are attached to the 

 abdominal wall by the mesogastrium (omentum) and mesenteries respec- 

 tively. These are thin, transparent membranes composed of loose connec- 

 tive tissue containing many adipose cells, lymphocytes and granular 

 leukocytes, and covered on their free surfaces by mesothelium. They con- 

 tain many blood and lymph vessels. 



Arteries enter and leave the intestinal walls through the mesentery. In 

 the submucosa they form a network which in the small intestine gives off 

 two kinds of branches, both of which enter the muscularis mucosae. Some 

 of the arterial branches supply the intestinal glands with dense capillary 

 networks, others supply the viUi. The latter enter the base of each villus 

 and form a capillary network which is in close proximity to the epithelium. 

 At the tip of the villus the capillaries collect into veins which have a parallel 

 course with the arteries. 



Liver and Pancreas 



The liver. — The liver consists of four main lobes: a large median, a right 

 and left lateral and a left caudal. All the lobes except the left lateral lobe 

 are partially divided by deep bifurcations. The gall bladder is attached to 

 the caudal surface of the median lobe. 



From the very thin connective tissue capsule covering the lobes, strands 

 of connective tissue project into the gland and form the supporting frame- 

 work, or capsule of Glisson, for the interlobular vessels and bile ducts. 

 This framework is very poorly developed and divides the gland into indis- 

 tinct polygonal areas, the lobules. In the center of each lobule is the cen- 

 tral vein, surrounded by anastomosing, radially arranged cords of liver 

 cells (Fig. 58). The central veins are intralobular tributaries of the hepatic 

 veins. The interlobular vessels which are surrounded by the capsules of 



