402 EXTERNAL RESPIRATION 



suspended. For example, in attempting to push a heavy body, 

 pull against an almost unyielding resistance, or lift a heavy weight, 

 a deep inspiration is taken and then, by powerful muscular action, 

 the whole body is knitted into a single lever with a long power arm. 

 The contracted abdominal muscles press the diaphragm, etc., up 

 into the thoracic cavity. This compression at first causes blood 

 to be squeezed from the abdominal veins into the thoracic veins 

 and so into the heart, producing a marked rise in arterial pressure. 

 Later there is a damming back of blood into the peripheral vessels, 

 causing them to stand out like knotted cords. When the effort 

 ceases and the compression on the abdominal contents is released, 

 the large abdominal and thoracic veins fill up with blood, producing 

 a marked fall in arterial pressure. The tissues during the period 

 of sustained effort have removed oxygen in large quantities from 

 the blood in the peripheral vessels, and if the effort is long continued 

 an oxygen debt is incurred (q.v.). Marked cyanoses may even 

 develop. 



Modifications of the Respiratory Act. A similar positive pressure 

 is brought to bear on the veins entering the thorax during such acts 

 as coughing, sneezing, defsecation and parturition. In the first 

 stage of coughing and in defsecation, after a forced inspiration, the 

 glottis {q.v.) is closed and the expiratory muscles put into strong 

 contraction. The enormous positive pressure produced, in the 

 former case, in thorax or abdomen, and, in the latter case, in the 

 abdomen, dams back the blood in the veins, causes a fall of arterial 

 pressure, and, if sustained, cyanosis follows. In sneezing, the air 

 is compressed after a forced inspiration by the contraction of the 

 pillars of the fauces, descent of the soft palate and pressure of the 

 tip of the tongue against the hard palate. The effect on blood 

 pressure is similar to that of coughing. Sighing and yawning, 

 which are alleged to be controlled by separate centres in the 

 medulla, are deep inspiratory acts almost entirely thoracic in 

 character and, therefore, tend to encourage diastolic filling and a 

 general increase of blood pressure. 



Influence of Heart Action on Respiration. 



As the heart contracts and dilates it nuist alternately decrease 

 and increase the intrapleural pressure, causing a slight inrush and 

 outrush of air with each cycle. These cardio-pneumatic move- 

 ments may be demonstrated in a very simple manner by filling 

 the mouth with tobacco smoke, inserting a glass tube about 

 18 in. by | in. in the mouth. Hold the tube vertically (preferably 

 with the upper end gently plugged with cotton wool). Allow a 

 little smoke to enter the tube and then hold the breath. The 



