1090 PHYSIOLOGY 



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 dimen- 

 sions, from above downwards by the contraction of the diaphragm, and 

 in its transverse diameters by the movements of the ribs. 1 



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 

 digit ations 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-surface of the 

 diaphragm by the elasticity of the abdominal walls. 

 The central part of the diaphragm is thus pressed 

 phragm in respiration. up into the chest, partly by the intra-abdominal 

 t, inspiratory position; pressure and partly bv the elastic traction of the 



e e, expiratory position. * . , .. . *__ " . . . , 



(YBO.) 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 lower surface of the lungs descends. The 

 enlargement of the lungs at the lower part of the thorax is aided by the 

 abduction of the floating ribs, produced by the contraction of the quadratics 

 lumborum and deep costal muscles. In this contraction the diaphragm 

 presses on the contents of the abdomen, so that the abdomen swells up 

 with each inspiratory movement. The middle of the central tendon, where 

 the heart lies, moves less than the two domes, and the part where the vena 

 cava passes through the tendon is practically stationary during normal 

 respiration. In deep inspiration however both this part as well as the rest 

 of the pericardial attachment is forcibly depressed towards the abdomen. 

 In quiet breathing, when observed by the Rontgen rays, the mean descent 



1 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). 



