1164 PHYSIOLOGY 



ments of the thorax, which constitute normal breathing. With 

 inspiration the cavity of the thorax is enlarged, and the lungs swell up to 

 fill the increased space. The capacity of the air-passages of the lungs 

 being thus increased, air is sucked in through the trachea. The move- 

 ment of inspiration is followed by that of expiration, which causes 

 diminution of the capacity of the thorax and expulsion of air. At the 

 end of expiration there is normally a slight pause. The number of respi- 

 rations in the adult is about 17 or 18 a minute. This is, however, much 

 influenced by various conditions of the body, and also by the age of the 

 individual. Thus a new-born child breathes about 44 times a minute, 

 a child of five about 26 times, a man of twenty-five about 16, and of 

 fifty about 18. The frequency is increased by any muscular effort, 

 so that even standing up increases the number of respirations. These 

 movements are much affected by psychical activity ; they are to a 

 certain extent under the control of the will, although they can occur 

 in an animal deprived of its brain, and are normally carried out without 

 any special act of volition. We can breathe fast or slow at pleasure, 

 and can even cease breathing for a time. It is impossible, however, to 

 prolong this respiratory standstill for more than a minute ; the need 

 of breathing becomes imperative, and against our will we are forced to 

 breathe. 



With every inspiration the cavity of the thorax is enlarged in all 

 dimensions, from above downwards by the contraction of the dia- 

 phragm, and in its transverse diameters by the movements of the ribs.* 



The diaphragm is a sheet separating the cavity of the chest from 

 that of the abdomen. It consists of a central tendon which forms an 

 arched double cupola, to the circumference of which are attached 

 muscle-fibres. The diaphragmatic muscles present two main divisions, 

 namely, (1) the spinal or crural part, the fibres of which arise from the 

 upper three or four lumbar vertebrae and from the arcuate ligaments 

 and are inserted into the posterior margin of the central tendon ; and 

 (2) the sterno-costal part, which arises by a series of digitations from 

 the cartilages and adjoining bony parts of the lower six ribs and 

 from the back of the ensiform process. These latter fibres pass back- 

 wards as they ascend. In the cavity of the larger dome on the right 

 side lies the liver, while the smaller dome on the left side is occupied 

 by the spleen and stomach. These viscera in the normal condition 

 are pressed against the under-surface of the diaphragm by the elas- 

 ticity of the abdominal walls. The central part of the diaphragm 

 is thus pressed up into the chest, partly by the intra-abdominal 

 pressure and partly by the elastic traction of the distended lungs. 



* The student is advised to consult the article by Keith on the ' Mechanism 

 of Respiration in Man ' for a fuller account of this subject (L. Hill's ' Further 

 Advances in Physiology,' 1909). 



