This power of the heart, so wonderful both in extent and duration, must be re ferred to the irritability of the organ, in which point of view it seems far to ex ceed all other muscular parts of the body. That the immediate cause of contraction in this viscus arises from the presence of blood in its cavities, is shewn by the cele brated experiment of Haller ; in which the longer duration of action in the right or left cavities, was varied by influencing the supply of blood.
In the action of those muscles that de pend on the will, a supply of nerves, and a distribution of blood to the moving fi bres, seem to be essential conditions. It has been disputed whether or not these circumstances are necessary in the heart, and what share they may contribute to the heart's action. We may observe in the first place, that the actions of the heart arc completely involuntary ; that no exertion of the will can produce the smallest effect in accelerating, retarding, or otherwise affecting the actions of this part. Yet various arguments prove that the nerves exert an influence over this organ. Not to mention the peculiar arrangement of the cardiac nerves, the sympathy between the heart's action, and nearly every Other function, even of the most different classes, suffices to demon strate the connection. The vehement disturbance of the heart from the passions of the mind, must be familiar to every person from his own experience; its ac tion is also strongly influenced by various states and affections of the alimentary canal.
The action of the heart is intimately connected with the changes which the blood undergoes in its passage through the lungs; for when is ob structed, the heart's action ceases, and it may be recalled by again introducing air into the lungs. Hence arises the impor tance of inflating the lungs in instances of apparent death from drowning, &c. in order to excite the heart to action. The mode in which the arteries and veins con tribute to the circulation, will be under stood from the account of those vessels in the article ANATOMY.
The circulation of the blood is different in the foetus, in consequence of differ enees in the structure of the organs de voted to this function. See FmTt-S.
The situation of the child in utero pre cludes the access of atmospheric air to the lungs ; these organs arc consequently small and collapsed, and the lesser cir culation can hardly be said to take place in the firtal state. Although its circulation might, in this respect, be considered as more simple than that of the adult, this function becomes considerably compli cated by the connection with the pla centa:. A portion only of the child's blood circulates through this part, and it is no doubt so altered or modified by this pas sage through the vessels of the placenta, -as to become more fit for the growth and nourishment of the child. No such altera tion or modification has, however, been actually demonstrated in the foetal blood. Physiologists have discovered no differ ence in this fluid in the various vessels of the foetus. It is of the same dark colour in the arteries and veins. The interrup tion of the communication with the pla centa, before respiration has commenced, is, however, suddenly fatal. Our igno rance of the functions of the placenta, and of the liver, which is of immense size in the foetus, as well as of the changes which the foetal blood undergoes in the compli cated system of organs, which arc con nected with its circulation in this state of existence, leaves many parts of the sub ject in doubt and obscurity.
Respiration. The exposure of the blood to the atmospheric air, by which the chyle, that has entered the circulating system from the thoracic duct, is con verted into blood, and by which those changes are effected in the whole mass of circulating fluid, which are essentially necessary to the continuation of life, takes place in the lungs.
The respiratory organ has been aptly compared to an empty bladder, placed ;n a pair of bellows, with its neck adapted to the instrument, and giving entrance to a column of air, when the sides are sepa rated. In breathing, the dilatation of the chest occasions the lungs to enlarge by the entrance of air into them from with out ; these viscera not possessing Ay means of enlargement in themselves this is' termed inspiration. The expulsion of the air, after it has served the purposes of respiration, by means of a process exactly contrary to the former, is called expiration. The diaphragm and the ab dominal muscles are the chief agents in enlarging and diminishing the chest. The former muscle in its relaxed state is strongly arched, and the convexity of this arch is towards the chest. Its curved fibres become straight by the contraction: the whole muscle descends towards the abdomen, and pushes the abdominal vis cera, which lie against its under or con cave surface, downwards and forwards. Hence the surface of the belly rises when we draw air into the chest. In the next moment, the abdominal muscles contract and push back the viscera, and thereby. diminish the chest in a degree propor tionate to its former enlargement. The, increase of the thorax, effected in this way, takes place in the perpendicular direction; hut it may also be enlarged in its whole diameter by means of the in tercostal muscles, which, by elevating and twisting outwards all the ribs, push the sternum forwards, and enlarge the chest in every direction. When the action of these powers has ceased, the natural elasticity of the parts restores the parietes of the thorax to their former position. In natural respiration, both these methods of altering the capacity of the chest are employed ; but females seem to use the intercostal muscles more than the male subject,, as the heaving of their bosom demonstrates ; yet breathing can be car ried on by either method, to the exclu sion of the other; as we sometimes see under cireamstances of accident or dis ease. In the case of a broken rib, where the rubbing of the broken ends would be highly painful, the chest is bound up so as to render the ribs motionless, and the diaphragm and abdominal muscles per form the whole business of respiration. When the diaphragm and abdominal muscles act together, they compress the viscera between them, and the pressure thus produced, assists in the expulsion of their contents. This effort is termed and is seen in vomiting, in the att. of discharging the feces and urine,.