THE RABBIT 277 



Note especially the heart, which will be seen to be inclosed in 

 the membranous pericardium ; anterior to it and partly covering 

 its base is the thymus, a soft, pale, pinkish body which is very 

 variable in size, being larger in young animals than in older ones. 

 Dorsal to the heart are two thin membranes, which pass diagonally, 

 to the right and left, to the dorsal body wall ; these are the medial 

 walls of the pleural sacs, and the space between them is the 

 mediastinum, in which the heart lies. Ventral to the heart the 

 walls of the mediastinum are much nearer to each other than 

 dorsally, and appear as a single membrane which joins the peri- 

 cardium with the body wall in the midventral plane. Do not 

 disturb any of these organs. 



Exercise 5. Draw a figure representing a diagrammatic cross sec- 

 tion through the thoracic cavity, showing the relative positions 

 of the two pleural sacs, the mediastinum, the heart with the peri- 

 cardium, and the lungs. 



The Digestive System. This system is made up of the mouth, 

 pharynx, stomach, intestines, salivary glands, liver, and pancreas. 

 Note the flat, dark-red spleen, which will be seen in the omentum 

 along the left side of the stomach, and also the splenic veins and 

 arteries. Do not cut any of these organs. 



Observe carefully the form and position of the stomach. Note 

 where the oesophagus comes through the diaphragm and joins it. 

 The anterior portion of the stomach is called its cardiac end ; the 

 posterior portion, where it passes into the intestine, is called the 

 pyloric end, and is marked by a constriction ; its convex posterior 

 surface is called the greater curvature, and its concave anterior 

 surface the lesser curvature. The portion of the greater curvature 

 which extends farthest to the animal's left is called the fundus of 

 the stomach. Raise the fundus and observe the mesogaster, the 

 fold of the peritoneum joining the dorsal abdominal wall and the 

 diaphragm with the stomach. 



Study the course of the intestine, but without cutting anything. 

 This portion of the digestive tract is made up of two divisions: 

 an anterior division, the small intestine, and a posterior division, 

 the large intestine. 



