and examine with a higher power. Note the difference in thick- 

 ness of the walls of the arteries, veins, and capillaries. The walls 

 of the latter appear merely as thin lines. Also note the rapid 

 stream in the middle of the arteries, the slower one near the walls ; 

 this latter may be seen to contain many leucocytes. The flow in 

 the capillaries is much slower ; the red corpuscles are often distorted 

 in shape, but recover again. They are thus elastic. 



The Heart of the Sheep 



Obtain a sheep's heart from the butcher, if possible with the 

 lungs attached. This is often called the " pull." 



The anterior or ventral surface is recognised by the groove filled 

 with fat, which marks the boundary between the two ventricles. 

 Note that the right ventricle is softer than the left. The pulmonary 

 artery comes from the former nearly in the middle line. The aorta 

 is behind it and will be cut. 



The two great veins from the body, superior and inferior venae 

 cavae, have been cut. Note that they enter the right auricle, so 

 that the blood passes into the right side of the heart and from the 

 ventricle into the lungs. Find the veins returning from the lungs 

 to the left auricle, their blood thence flowing to the left ventricle 

 and to the aorta. 



Open the right auricle and cut away most of it. Pour water 

 into the orifice leading to the right ventricle. Note the flaps of the 

 tricuspid valve floating up. 



With scissors, one blade between two of the flaps of the valve, 

 cut through the wall of the ventricle towards the apex. Then 

 upwards again along the septum, but not quite into the pulmonary 

 artery. Cut across the pulmonary artery, and tie into it a glass 

 tube a few inches long and as large as will fit in. Pour water into 

 this tube, and observe from below the closed semilunar valves. 

 Note also the fine cords attaching the flaps of the tricuspid valve 

 to the walls of the ventricle. 



Open up the left auricle and ventricle in a similar way, noting 

 especially the openings of the arteries feeding the heart (coronary 

 arteries) behind the aortic valves. 



Look for the band of special conducting tissue passing from 

 auricles to ventricles. Branches will be found on both sides of the 

 septum between the ventricles. They stain deeply with iodine on 

 account of the large amount of glycogen they contain. 



Blood 



Mount quickly a small drop of blood from the finger. Examine 

 at once with a high power. Note : 



