60 PRINCIPLES OF PHYSIOLOGY 



blood going to a part is regulated, movements 

 in certain ducts, as in the ureter (the duct 

 passing from the kidney to the bladder), the 

 contractions of the skin, are all effected by 

 non-striated muscle. The contractions of 

 cilia create a current in one direction, as in 

 those in the respiratory passages causing a 

 movement of air and mucus towards the 

 opening of the respiratory passage. The 

 leucocytes, by their contractions, can pass 

 out of the vessels into surrounding tissues, 

 and there they may seize hold of, kill, and 

 digest foreign organisms, such as many bacteria 

 and bacilli that cause disease. Probably, also, 

 by this power of ingesting foreign bodies, 

 they take part in processes of nutrition. All 

 contractile tissues, except the isolated cells, 

 are related to the central nervous system by 

 nerves that pass to and from these centres 

 to the contractile tissues. They are also 

 very richly supplied with blood vessels, and 

 they are nourished by the fluid that passes 

 out of the vessels, and bathes every fibre. 

 This fluid, called lymph, carries away all 

 excess of fluid, and also many substances in 



