PHYSIOLOGY FOR BEGINNERS 



ture of the air expired suddenly falls, as it does on a frosty 

 morning, some of the aqueous vapour in it condenses to a 

 cloud, or, if breathed on to a cold surface such as a window 

 pane, to minute particles of water. The expired air also con- 

 tains organic substances in very small amount ; the import- 

 ance of these is that they are injurious if breathed in again. 

 The amount of nitrogen is usually unchanged, but sometimes 

 there is a very little more, sometimes a little less, in the 

 expired air than in the inspired air. Thus the body by the 

 lungs gains oxygen, and loses carbonic acid and water. When 

 oxygen unites with carbon the carbonic acid formed occupies 

 the same volume as the oxygen consumed, so that if all the 

 oxygen taken in by the lungs was used in this way, the amount 

 of carbonic acid formed would be exactly equal to it. The 

 volume of carbonic acid, however, given off by the lungs is 

 always a little less than the oxygen taken in. This shows 

 that the oxygen is used for other purposes than merely uniting 

 with carbon. The oxygen is taken up by the tissues, and as 

 they break down most of it unites with the carbon of the 

 tissues, but some of the oxygen unites with the hydrogen of 

 the tissues to form water, and some of the water so formed 

 leaves the body by the lungs as aqueous vapour. A very small 

 amount of the oxygen unites with other elements, such as 

 sulphur. 



The Respiratory Organs 



The Upper Air Passages. On its way to the lungs the 

 air passes through the mouth or nose, the pharynx, the larynx, 

 and the trachea. The sides of the cavity of the mouth are formed 

 by the cheeks, the floor by the tongue, and the roof by the 

 palate, the partition separating the cavity of the mouth from 

 that of the nose. The front of the palate is hard (the hard 

 palate) from the presence of a plate of bone ; the hinder part 

 is soft (the soft palate) and consists of a thin sheet of muscle, 

 covered like the rest of the lining of the mouth by a layer 

 of tissue called mucous membrane. The soft palate, which 

 carries in the centre a prolongation, the uvula, meets the side 

 walls of the mouth at the pillars of the fauces, as they are 

 called, between which the tonsils are placed. The soft 



