DIAPHRAGM. 



above the tendon ; but when brought into ac- 

 tion, each fibre approaches to a right line, which 

 runs obliquely down from the tendon to its 

 point of insertion. Thus, instead of a great 

 arch we have a number of inclined planes, very 

 short in front, very long at the sides, and of 

 intermediate length further back, all surmount- 

 ed by a tendinous platform. The base of the 

 lung resting on the muscle descends with it; 

 the liver, stomach, spleen, and all the moveable 

 viscera of the abdomen are pressed downwards 

 and forwards against the abdominal muscles. 

 When the diaphragm descends, therefore, in- 

 spiration takes place by the rush of air into the 

 expanding thorax ; when it ascends expiration 

 is the result, the air being forced out. In the 

 former case the diaphragm is active; in the 

 latter it is completely passive, following the re- 

 siliency of the lungs, and pressed up by the 

 action of the abdominal muscles on the viscera 

 beneath.* The central tendon descends very 

 little on account of its attachment to the peri- 

 cardium: descent here would be useless or 

 worse; but the lateral portions on which the 

 broad bases of the lungs rest, freely change their 

 place, and allow of considerable expansion of 

 the thorax where it is most required. 



From viewing the insertion of the diaphragm 

 into the lower ribs it might be thought that 

 they would be drawn in by its action, and the 

 capacity of the thorax thereby diminished more 

 than increased; but the intercostals prevent 

 this occurrence by acting at the same moment 

 to elevate and draw out the ribs. 



The crura, besides acting in common with 

 the broad muscle in enlarging the thorax, serve 

 to fix the central tendon, and prevent it from 

 being drawn to either side by the irregular ac- 

 tion of either half of the muscle, or forced too 

 high up. They may also by their fibres conti- 

 nued on each side of the cesophageal orifice, 

 and contracting in concert with the rest of the 

 muscle, close that opening, and thus prevent 

 regurgitation from the stomach at the time 

 when this viscus is pressed upon by the descent 

 of the diaphragm. 



The extent to which the diaphragm descends 

 is not great. The central tendon will not admit 

 of much displacement in the normal state of 

 the parts, and the shape and motions of the 

 liver show that even the great alae do not un- 

 dergo much alteration. Haller indeed says 

 that he saw the diaphragm descend so much in 

 violent inspiration as to present a convexity to- 

 wards the abdomen.f But this is quite incre- 

 dible. The utmost muscular effort, if there 

 were no fixed point in its centre, could only 

 obliterate the arch ; but even this we think im- 

 possible on account of its attachments. We 

 find on some occasions one side of the dia- 

 phragm act independently of the other. 



The importance of the diaphragm in respira- 

 tion is shewn by the difficulty with which that 



f ~ * Senac says the anterior fibres assist in expira- 

 tion by drawing the ribs inwards and backwards. 

 Acad. des Sciences, 1729. 



f In violentissima rrspiratione oranino vidi deor- 

 sum versus abdomen diaphragma convexum reddi. 

 Haller, Elfin. Phys. lib. viii. sect. 1. 



function is performed when the actions of the 

 muscle are interfered with. Ascites and tu- 

 mours in the abdomen render the breathing 

 shorter; even a full meal will have this effect, 

 owing to the impediments to the descent of the 

 diaphragm. If the phrenic nerves be divided 

 in a living animal, great difficulty of breathing 

 follows, the entire labour of respiration being 

 thrown on the muscles which elevate the ribs. 

 If the spinal marrow be divided above the 

 giving off of the phrenic nerves, respiration 

 ceases at once, but not so if divided imme- 

 diately below that point ; and in a case of fatal 

 dyspnoea Beclard could find no cause but a 

 tumour on one of the phrenic nerves. 



Besides the part which it plays in respira- 

 tion, it is probable that the diaphragm, by its 

 ordinary motions, exerts a beneficial influence 

 on the digestive organs. The liver must be 

 more or less affected by it in its secretion, and 

 the gall-bladder is supposed to receive from it 

 a compression which in some degree makes 

 amends for the want of muscular fibres,* whilst 

 the agitation of the hollow viscera will favour 

 the transmission of their contents. 



The chyle in the lacteals and thoracic duct 

 may also receive an impulse from the dia- 

 phragm. 



Some anatomists were of opinion that the 

 venous circulation in the abdomen was also 

 assisted by the pressure, but the absence of 

 valves in these vessels must prevent them from 

 deriving any assistance from alternate compres- 

 sion and relaxation. It acts powerfully, how- 

 ever, on the venous circulation of the whole 

 system by the vacuum which it has a tendency 

 to form in the thorax. 



The nerves which pass through the dia- 

 phragm, as the par vagum, sympathetics, and 

 splanchnics, were formerly supposed to suffer 

 compression, and the alternate transmission and 

 interruption of the nervous influence, it was 

 thought, could account for the pulsations of 

 the heart and the vermicular motions of the 

 intestines. But all this is too obviously erro- 

 neous to require comment. 



The diaphragm assists, though rather as a 

 passive instrument, in the expulsion of the 

 urine, faeces, &c. For this purpose the thorax 

 is filled with air, the rima glottidis is closed, 

 and the diaphragm forms a resisting surface 

 against which the abdominal muscles press the 

 hollow viscera, and force out their contents 

 wherever an exit is afforded them. 



The diaphragm is more or less engaged in 

 hiccup, yawning, sighing, sobbing, groaning, 

 which are all actions connected in various 

 ways with the function of respiration, and some 

 of them more especially dependent on the dia- 

 phragm, particularly hiccup, which is an explo- 

 sive inspiration, in which the diaphragm acts 

 involuntarily by a short and sudden effort, a 

 sound being at the same time produced in the 

 larynx. 



The diaphragm also performs an important 

 part in vomiting. A full inspiration precedes 

 this act, then the glottis is closed, and the ab<- 



'* St-nac. 



