But without entering at length into this question, which will be more fully discussed under the head of Respiration, it is obvious from what has been said, that provision must be made, in the ce con omy of all living beings, for the exposure of their fluids to the air of the atmosphere, in circumstances admitting of ex halation and absorption ; and it may be. farther stated, that, in the different classes of animals, the amount of this mutual action for which provision has to be made, must be proportioned to the energy and activity of vital action which each animal is destined to exhibit, these qualities being very generally found to be greater, as the consumption and vitiation of the air are more rapid.§ These principles explain the intention of many different contrivances and arrangements, afterwards to be described, which are em ployed in different classes of animals for the performance of the function of respiration; and the variations of which may be said, in a gene ral view, to be determined by two conditions, first by the medium in which each animal is destined to exist, and secondly, by the inten sity and variety of vital actions which it is to be capable of performing.
The importance, to all living beings, of the action of oxygen on their fluids is most un equivocally shewn by the nature of the fatal changes which ensue, when that action is in any way obstructed ; i. e. by the nature of the changes which take place in death by asphyxia. The study of these has long been held to be of the highest importance, not only as a car dinal point in physiology, but as affording the only precise information in regard to the tendency of many and various diseases.
It is chiefly in animals of the highest orders, i. e. in warm-blooded animals, that these phe nomena have been studied ; and it is to be remembered, that in them the subject is ren dered more complex by the higher endow ments and greater power over all functions of the body,which the nervous system there possesses. When we trace the connection, in these animals, of the different changes that precede the fatal event, it is right to bear in mind, that the in terruption of the process by which their fluids are exposed to the air is equally fatal, not only to those animals in which no action of the ner vous system is concerned in that process, but also in vegetables, where no nervous system exists.
The phenomena of asphyxia in the higher animals are very nearly the same, in whatever manner the access-of air to the organs of respi ration is prevented. This may be done, in the case of animals that breathe by lungs, in a great variety of ways ; by strangulation or suf focation, i. e. by any mechanical means pro hibiting the ingress of air by the trachea and bronchi ; by submersion in water or any other fluid ; by confinement in vacuo or in such gases as contain no oxygen, but are not them selves poisonous, such as azote and hydrogen; by forcible compression of the thorax, prevent ing its dilatation ; or by the admission of air into free contact with the surface of the lungs on both sides of the chest, so as to prevent their distension, as in the celebrated experiment of Dr. Hooke ; or by the section, either of all
the separate nerves which move the muscles concerned in the dilatation of the thorax in inspiration, or of the spinal cord in the upper part of the neck, above the origin of the phrenics, by which the whole of these nerves are simultaneously palsied, as in many ex periments of Galen, Cruikshank, Le Gallois, and others.* In the case of fishes or other animals that breathe by gills, where several of the methods above enumerated are inapplicable, asphyxia is produced, either by confinement in air, or in distilled water, or water impregnated with any gas that does not contain oxygen ; for no ani mal has the power of decomposing water by its organs of respiration, to obtain oxygen, and all aquatic animals are dependent, either on the occasional respiration of atmospheric air by lungs, or on the more constant respira tion of the air contained in water by gills or analogous organs.
In the case of fishes breathing by gills, as the motion of these organs is dependent on nerves arising as high as the medulla oblongata, injury of the nervous system must be as high as that part, in order to produce asphyxia ; and on the other hand, in the case of birds, where the expansion of the thorax in inspira tion is effected almost entirely by the motion of the ribs, asphyxia may be produced by section of the spinal cord in any part of the iieck.' We exclude here entirely the cases, often described under the name of asphyxia, in which gases positively noxious (such as carbonic acid, carburetted hydrogen, 84c.) have been breathed, because accurate observation shows that these are in fact cases of poisoning, where the poison has been introduced by the lungs, and not simply cases of asphyxia.
The phenomena of asphyxia, in all the cases above-mentioned, (as occurring especially in the warm-blooded animals,) may be divided into three stages. The first is characterized by the intensity of the sensation which prompts to acts of inspiration, and the consequently violent and laborious, though ineffectual attempts to appease that sensation by the action of all the muscles of inspiration ; and in some in stances by other actions, voluntary or instinc tive, but still under the guidance of sensibility. Lividity of the surface takes place before the end even of this stage. The next is distinguished by insensibility, rapidly increasing, and attend ed with irregular spasms or convulsions ; and the last by cessation of all effort, and of all outward signs of life, while the heart's action and circulation are known still to go on for a short time.