THE CIRCULATION 223 



problem is presented of which no completely satisfactory 

 solution has yet been given. How comes it that lymph is not 

 sucked into the pleural cavity ? In health there is no more 

 pleural fluid than just suffices to keep the membrane moist. 

 The endothelial cells which cover the surface of the pleura resist 

 further exudation. Valves in the lymphatic vessels prevent 

 backward flow. Yet in disease, when the pleura is inflamed, 

 lymph pours out quickly, often to be reabsorbed with equal 

 rapidity when the pleurisy subsides. This flow uphill, from a 

 lower to a higher pressure, can be explained only as a pheno- 

 menon due to the " secretory " capacity of endothelium. As 

 an answer to the hydrostatic problem this is hardly satisfactory. 



The circulation of the blood is the result of the difference 

 between the pressure in the vessels through which it leaves the 

 heart, and that in the vessels through which it is returned. 

 The pressure in the aorta amounts to about 200 millimetres 

 of mercury. In the venae cavae it is nil, or, owing to the 

 aspiration of the thorax, less than nil. 



The Heart. Inspection of the liver, the spleen, or the kidney 

 helps but little to the comprehension of the mechanism of these 

 organs. It is quite otherwise in the case of the heart. Its 

 mechanics being comparatively simple, physiology is concerned 

 with measurements, with the conditions under which it can 

 and cannot work, and with the action upon it of the nervous 

 system and of drugs. The heart of any mammal will suffice 

 for anatomical study. A sheep's heart is about the same size 

 as that of a man, and exactly similar, save in minute par- 

 ticulars, which do not appreciably affect its mode of working. 



The heart is a hollow muscle, composed of minute contractile 

 cells. Each cell is a cylinder, about twice as long as it is broad, 

 with an oval nucleus in its centre. There is no impropriety in 

 speaking of the heart as a single muscle. Muscles which we can 

 move at will, " voluntary muscles," consist of fibres, each from 

 1 inch to 2 inches long, and of about the thickness of a piece of 

 thread (Fig. 16). Every fibre is surrounded by a membranous 

 sheath, its sarcolemma, which completely isolates it from the 

 others. Each has its separate nerve-supply. A voluntary muscle- 

 fibre is a cell-complex. The single embryonic cell which grew 

 into the fibre underwent nuclear division until hundreds of 

 nuclei were formed, but its cell-substance was not divided into 



