I0 4 PHYSIOLOGY FOR BEGINNKKS n\i' 



If the ear is gently rubbed fora few moments, it will become 

 very red. This is due to a dilation of the blood-vessels of the 

 ear, and consequently a large supply of blood to the ear, and 

 this may be brought about by the direct action of the handling 

 on the vessels themselves without the intervention of the central 

 nervous system. The blood-vessels of many organs can thus 

 be locally influenced by various agencies. 



There are thus three ways in which the blood supply may 

 be regulated : 



1. Alteration in the heart's beat, leading to a change affect- 



ing all parts of the body. 



2. By the action of the vaso-motor nervous impulses on the 



small vessels of particular parts, so as to lead to a 

 change of their calibre. 



3. By direct action on the muscle fibres of the small vessels, 



leading to a dilation of the vessels. 



The Lymphatic Circulation 



The blood circulates in a system of tubes, the smallest of 

 which, the capillaries, by their close meshwork in the tissues, 

 bring it very close to the individual cells. Still the blood is 

 confined to the tubes. The walls of the blood capillaries are, 

 as we have seen, exceedingly thin, being formed of one layer 

 of flattened cells joined edge to edge. Part of the plasma of 

 the blood passes through the thin walls of the capillaries, and 

 so brings the nutritive material actually to the cells. This fluid, 

 which exudes from the blood-vessels and bathes, as it were, 

 the actual tissues, is called the lymph. 



Lymph. Lymph is a colourless fluid, like blood plasma 

 in composition, that is, consists of water containing proteids, 

 salts, and other substances in solution, but in rather less 

 quantity. When shed it clots like blood plasma. The 

 lymph contains colourless corpuscles like those of the blood, 

 but no red ones. 



Lymphatic Vessels. The lymph lies in spaces which 

 exist between the cells of the tissues, and these spaces are 

 drained by a network of delicate vessels, called lymphatic 

 vessels, which gradually unite with one another, forming a few 

 main lymphatic vessels by which the lymph is carried away 



