Human Physiology. 209 



curious force of membranous circulation called endosmose and 

 exosmose. The heart cannot draw the blood back through the 

 veins their flabby structure would collapse at once. The 

 blood must be pushed back to the heart by the capillary forces. 



But the whole circulation is far from being entirely mechani- 

 cal it is far more nervous and vital. The heart is supplied 

 with blood-vessels, arteries, and veins for its own nourishment. 

 So are the arteries and veins. The nerves of organic life per- 

 vade them everywhere. When a draft is made upon the brain 

 for an extraordinary effort, not only is the action of the heart 

 quickened, but the arteries enlarge, and a fuller current goes to 

 the head, and is seen in the flushed face, while the extremities 

 become cold. Blood goes where it is most wanted. The heart 

 can only pump away like a fire-engine it has no power to 

 direct the current ; but the nervous power can send it, now to 

 the brain for thought, now to the stomach for digestion, then 

 to the muscles, then to heal a broken leg. When a leg is 

 broken, the arteries and veins expand around the fracture; 

 bony matter is deposited, first in a ring around and a plug in 

 the hollow of the bone then the ends are neatly joined 

 together, and then the superfluous bony matter the staging 

 is carried away; and all this is done by the nervous power 

 controlling the small blood-vessels by the intelligent powers 

 of life. 



If the circulation is governed by the organic nerves, they 

 are subject to many influences from the cerebro-spinal system. 

 Joy quickens the pulse grief depresses it. A sudden emotion 

 will fill the capillaries of the skin in the face and neck with red 

 globules, as is seen in the lovely phenomenon of blushing. 

 Under another emotion the arteries of the bosom fill, and the 

 glands transform blood into milk. Anger sometimes colours the 

 blood and thence the skin with bile. Fear or anxiety will in- 

 crease another secretion from the blood, and cause perspiration. 

 If an arterial trunk supplying leg or arm be tied, nearly all 



