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 sonic 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 doininal muscles forcibly press the stomach against the diaphragm, so as to assist the anti peristaltic motion of that viscus. Magendie made experiments to show that unless the dia phragm or abdominal muscles acted on the stomach, no vomiting could take place. Ile went too far, however, when he attributed the entire result to them. Substituting a pig's bladder for the stomach, he injected tartar emetic into the veins, and vomiting followed.
But he forgot that pressure might readily empty a dead bladder and have little effect on a living stomach. And that such is the case we may be certain, else every cough would evacuate the stomach.
Lastly, the diaphragm acts the part of a sep tum or inediastinum to separate the two great cavities between which it is placed. When this septum is wanting, the abdominal viscera get into the thorax, and in such cases the lungs are constantly found in a rudimentary state : their further evolution being impeded by the pressure exerted on them by the intruding viscera.* It has been stated that the cesophageal open ing may be closed by those fibres of the crura which curve round it. The other openings, as the aortic, and that for the ascending eava, can not be diminished by the efforts of the muscle. This is plain from the tendinous margins which they present, and the manner in which the mus cular fibres are attached to their borders.
We have not mentioned some of the uses which the ancients ascribed to the diaphragm— as, that it is the seat of the passions,j- that it prevented noxious vapours from rising into the thorax, that it fanned the hypochondria, and so forth. These are too fanciful to demand serious notice.
_Malformations and diseases. — The dia phragm may be absent in whole or in part by congenital malformation. In the very young foetus the thorax and abdomen form one cavity, as in birds, reptiles, and fishes; and the deve lopment of the diaphragm, as of most other organs, is by a process of growth from the cir cumference to the centre. If, therefore, an arrest of formation occur at a very early period of foetal existence, the muscle may be entirely wanting; if at a later period, sonic deficiency will be found at or near the centre. An exam ple of the total absence of the diaphragm was dissected by Diemorbroeck. The subject lived to the age of seven years without suffering any inconvenience except a frequent cough.t Con genital deficiencies near its middle are not very rare. They are observed oftener towards the left than the right side, and are always accom panied with a protrusion of the abdominal viscera into the thorax, not vice versa. The development of the thoracic viscera is impeded by this intrusion, and they remain more or less rudimentary. It sometimes happens that the natural openings of the diaphragm are too large, and then protrusions or hernia: are apt to occur by the sides of the tubes which they were intended alone to transmit.