MECHANICS OF RESPIRATORY MOVEMENTS 1041 



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 down- 

 wards by the contraction of the diaphragm, 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 con- 

 sists of a central tendon which forms an arched 



double cupola, to the circumference of which are FIG. 486. Diagram show- 

 ___. . ing movements of dia- 



attached muscle-fibres. The diaphragmatic muscles phragm in respiration. 



present two main divisions, namely, (1) the spinal *'', inspiratory position ; 

 or crural part, the fibres of which arise from the ( Y e E ' .f pir< 

 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 backwards 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- surf ace of the diaphragm by the elasticity 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 upper surface of the central tendon is 

 united to the pericardium. This part, during expiration, is the deepest part 

 of the middle portion of the diaphragm. Towards the back of the pericardial 

 attachment the central tendon is pierced for the passage of the inferior vena 

 cava. In expiration the lateral muscular zone of the diaphragm lies in 

 contact with the lower part of the thoracic wall. During inspiration the 

 muscle fibres contract and draw the central tendon downwards, so that the 



* 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). 



