126 A TEXTBOOK OF PHYSIOLOGY 



e.g., they may withstand an internal pressure up to about 

 10 atmospheres. It has proved possible to stitch divided arteries 

 and veins together so perfectly that the circulation can continue 

 through them. The kidneys have thus been successfully trans- 

 planted from one cat to another, and have continued to functionate 

 for some days. A piece of artery, killed by immersion in formol 

 solution, has been intercalated in the aorta of an animal, and the 

 circulation has continued unimpaired. It forms a scaffold for repair. 



The heart is enclosed in a tough inextensile bag the pericardium 

 the functions of which are to give the heart a smooth bag to 

 work in, moistened with pericardial fluid; to prevent misplacement 

 and check over-dilatation of the heart, in particular during great 

 muscular efforts. The pericardium restrains the over-stretching of 

 the heart in just the same way as the leather cover of a football stops 

 over-distension of the indiarubber bladder within it. 



The abdominal organs and bloodvessels, encompassed by the 

 muscular wall of the abdomen, may be regarded as enclosed in 

 a sphere of muscle. Above is the dome of the diaphragm, below 

 the basin-like levator ani, closing the outlet of the pelvis; in 

 front are the recti muscles, behind the quadrat! lumborum and the 

 spine; while the oblique and transverse muscles complete the wall 

 at either side. The brain is enclosed in a rigid and unyielding box 

 of bone the cranium; the limbs are encompassed by the extensile 

 and, in health, taut and elastic skin; while the organs, such as the 

 salivary glands and kidneys, possess a capsule which confines them 

 and limits .their expansion. 



The bloodvessels are thus confined by the walls and membranes 

 of the body and influenced by every muscular movement. 



The heart's energy is spent in maintaining a pressure of blood 

 in the elastic arteries, and owing to the difference of pressure in the 

 arteries and veins, the blood is kept flowing through the capillaries 

 into the veins. The movements of the body, particularly those of 

 respiration, help to return the blood from the capillaries and veins 

 to the heart. In the veins, especially those of the limbs, valves are 

 placed to direct the blood heartwards. The blood is propelled by the 

 heart, which varies both in rate and energy of beat, through muscular 

 and labile arteries, delicate capillaries, and muscular veins a system 

 which varies in capacity and may alter in lability. This system is 

 supported by the tissues which, by their contractility, elasticity, and 

 secretory force modify their support of the vascular system most 

 profoundly. The width of bed through which the blood flows varies 

 greatly at different parts of the circuit. The resistance offered to the 

 moving blood is very much greater in the capillary-sized vessels than 

 in the large arteries and veins. 



The problems of the circulation are thus far from simple. They 

 resolve themselves mainly into a consideration of (1) the physiology 

 of the heart; (2) the physical characters of the circulation; (3) the 

 control of the heart and vessels by the nervous system. 



