220 ELEMENTARY PHYSIOLOGY less. 



from view and the ear will become pale and cold. This 

 result we have already seen to take place (p. 67) when 

 the cervical sympathetic nerve, which supplies the 

 muscular wrapping of these vessels, is stimulated. 

 Indeed, when we analyse the action of the adrenalin we 

 find that it does actually stimulate the branches of the 

 cervical sympathetic not along the course of the fibres, 

 but at their termination just where they penetrate the 

 muscular fibres. Nor is the action of adrenalin confined 

 to nerve endings of sympathetic fibres going to blood- 

 vessels, but it stimulates the nerve endings of sympathetic 

 fibres generally, whatever their function. It therefore 

 causes acceleration of the heart,, secretion of saliva and 

 many other changes. 



The amount of adrenalin necessary to produce these 

 effects is very small. In a rabbit a rise of blood-pressure 

 may be obtained by the injection of one millionth of a 

 gramme. The amount of adrenalin injected, therefore, bears 

 about the same proportion to the rabbit as Avould a drop 

 of water dropped into an express locomotive engine, and 

 this suffices not only to construct every muscular fibre in 

 the vascular system, but to produce a multitude of other 

 effects. 



19. The Thymus Gland. — This is an organ which lies 

 over the trachea, in the lower part of the neck and 

 behind the sternum at the base of the heart. It is con- 

 spicuous at birth but soon begins to waste away, and in 

 the adult is replaced by a small amount of connective 

 tissue and fat. In structure it somewhat resembles a 

 lymphatic gland ; thus it has an external capsule from 

 which- trabeculie pass inwards and divide it up into 

 regular compartments or follicles. These follicles are 

 filled with a network of lymphatic connective tissue which 

 is crowded with leucocytes. 



Nothing very definite is known of the function or use 

 of this gland. 



20. The Spleen. — The spleen lies in the abdominal 



