THE DlArilRAGir. 9.55 



internal pectoral, derived from the anterior aorta ; but more from tlic pos- 

 terior intercostals which spring from the posterior aorta. 



The veins of the diaphragm belong exclusively to the posterior vena 

 cava. There are usually three on either side ; but they may be best referred 

 to two chief trunks which come from the circumference of the diaphragm, 

 converge towards the centre, and run into the posterior cava as it passes 

 through the tendinous expansion. 



The functional nerve of the diaphragm, or that from which it derives its 

 principal action, and which constitutes it a muscle of respiration, is the 

 phrenic or diaphragmatic. Although it does not proceed from that portion 

 of the medulla oblongata which g-ives rise to the glosso-pharyngeus and the 

 par vagum, yet there is sufficient to induce us to suspect that it arises from, 

 and shoukl be referred to, the lateral column between the superior and in- 

 ferior, the sensitive and motor nerves, and which may be evidently traced 

 from the pons varolii to the very termination of the spinal chord. 



The diaphragm is the main agent in the work of respiration. The other 

 muscles are mere auxiliaries, little needed in ordinary breathing, but aHbrd- 

 ing the most important assistance, when the breathing is more than usually 

 hurried. The mechanism of respiration may be thus explained : — Let it 

 be supposed that the lungs are in a quiescent state. The act of expiration 

 has been performed, and all is still. From some cause enveloped in mys- 

 tery — connected with the will, but independent of it — some stimulus of an 

 unexplained anduid^nown kind — the phrenic nerve acts on the diaphragm, 

 and that muscle contracts ; and, by contracting, its convexity into the chest 

 is diminished, and the ca\^ty of the chest is enlarged. At the sametime, 

 and by some consentaneous influence, the intercostal muscles act ; with no 

 great force, indeed, in undisturbed breatliing ; but, in proportion as they 

 act, the ribs rotate on their axes, their edges are thrown outward, and thus 

 a twofold effect ensues ; the posterior margin of the chest is expanded, the 

 cavity is plainly enlarged, and also, by the partial rotation of every rib, the 

 cavity is still more increased. 



By some other consentaneous influence, the spinal accessory nerve like- 

 wise exerts its power, and the sterno-maxillaris muscle is stimulated by 

 the anterior division of it, and the motion of the head and neck corresponds 

 with and assists that of the chest ; while the posterior division of the ac- 

 cessory nerve, by its anastomoses with the motor nerves of the levator 

 humeri and the splenius, and many other of the muscles of the neck and 

 the shoulder, and by its direct influence on the rhomboideus, associates 

 almost every muscle of the neck, the shoulder, and the chest, in the expan- 

 sion of the thorax. These latter are muscles which, in undisturbed respi- 

 ration, the animal scarcely needs ; but which are necessary to_ him when 

 the respiration is much disturbed, and to obtain the aid of which he will, 

 under pneumonia, obstinately stand until he falls exhausted or to die. 



The cavity of the chest is now enlarged. But this is a closed cavity, and 

 between its contents and the parietes of the chest a vacuum would be 

 formed; or rather an inequality of atmospheric pressure is produced from 

 the moment the chest begins to dilate. As the diaphragm recedes, there 

 is nothing to counterbalance the pressure of the atmospheric air com- 

 municating ^vith the lungs through the medium of the nostrils, and it 

 is forced into the respiratory tubes already described, and the lungs are 

 expanded and still kept in contact with the receding walls of the chest. 

 There is no sucking, no inhalent power in the act of inspiration ; it is the 

 simple enlargement of the chest from the entrance and pressure of the air. 

 From some cause, as inexplicable as that Avliich produced the expansion 

 of the chest, the respiratory nerves cease to act ; and the diaphragm, by 

 the inherent elasticity of its tendinous expansion and muscular fibres, re- 



