Journal of Agriciiliiin 



[lo Sept., 1908. 



so that their edges nearly meet. Then comes the systole. The contractiorb 

 in the veins has little action, but the quick snap of the auricle empties 

 this chamber, part of the blood passing back into the veins but the main 

 part being driven into the ventricle. Then the ventricle contracts strongly ; 

 the walls approach one another and exert pressure on the blood between 

 them. The mitral valve closes tightly, and the chordoe tendinece being 

 pulled on by the muscular pillars to which they are attached not only 

 prevent the mitral valve from being driven into the auricle, but actually 

 pull the whole valve and its attachment down into the ventricle, so that 

 it may be said that the walls on every aspect of the ventricle approach 

 each other in systole. Now out of the left ventricle arises the great 

 artery of the body, the aorta, and as the blood within it is under pressure 

 the similunar valves are tightly closed. When, however, the pressure in 

 the ventricle exceeds the pressure in the aorta, the semilunar valves open 

 and blood is shot out of the ventricle into the great artery ; then the 

 ventricle stops, and at the same instant the semilunar valves close tightly 



P'ig. 51. Diagram to show action of heart valves. A, left auricle; V, left 

 ventricle; Ao, aorta; P, muscular pillars attached to t, chordse tendinese; Z Zl, flaps- 

 of mitral valve. In the diagram to the left the heart is in diastole; the mitral 

 valve is open and the semilunar is shut. In the diagram to the right, the heart is in 

 systole, the mitral valve is shut whilst the semilunar is open. (After Schultz.) 



again. From the aorta arise the main arteries for supplying red arterial 

 blood to the whole body. This great vessel soon alter leaving the heart 

 forms an arch and then passes lengthwise through the body. In its 

 course it gives off numerous branches. The first branch is a tiny vessel 

 for the heart itself; then come large arteries for the fore limbs and head; 

 then small arteries for the walls of the body cavities; then as the aorta 

 passes through the abdominal region a mighty system leads to the abdominal 

 alimentary canal ; then a pair of vessels to the kidneys ; then arteries to 

 the pelvic organs, the lower limbs and tail. All these arteries give off 

 branches, and these again divide into smaller branches, and so on until 

 minute arterioles of microscopic size are formed. As the calibre of 

 each parent branch is less than the sum of the calibres of the smaller 

 branches it gives rise to, the total cross-section of the arterial system 

 gradually increases from the heart to the arterioles, and, in consequence, 

 the rate of the blood stream gradually diminishes. A change in the 

 structure of the walls may also be noted. The aorta and large arteries 

 have walls which are rigid with fibrous tissue and have only a little " give." 

 The smaller arteries have more elastic walls, whilst the arteriole walls- 

 are chieflly muscular, the muscles, by their contraction or relaxation. 

 constricting or dilating the calibre of these vessels. 



