It is thus that those persons perish, who experience any great lesion of the spinal marrow, between the brain and the origin of the phrenic nerves. Physicians have been very much embarrassed, in fixing with precision the spot, when a wound of the medulla ceases to be mortal; from what I have advanced, the limit is easily assigned.[113] From the same causes, concussion, and compression of the brain, are also fatal. We should observe notwithstanding, that these different causes of death, may act with various degrees of intensity. If they act but feebly, they affect the intellectual functions only, for these functions are always the first to be altered, in all lesions of the brain however small. If the lesion be greater, the affection extends to the muscles of the limbs, and convulsion or palsy ensue. Lastly, if the lesion be very great, the whole of the muscles of the animal life, the intercostals and diaphragm, as well as the others, are paralyzed, and death follows. We now can reply to the question proposed at the beginning of this section, and affirm that the death of the lungs is occasioned indirectly, by the death of the brain. It follows also, from the principles which are above established, that respiration is a mixed function, a function placed as it were between the two lives, to which it serves as a point of contact, belonging to the animal life by its mechanical functions, and to the organic life, by its chemical functions; and hence we have the reason no doubt, why the existence of the lungs is as much connected with that of the brain, as with that of the heart. It may be observed in the series of animals, that in proportion as the organization of the brain is straitened, a number of the phenomena of respiration also are lost. In birds, and the mammalia, this function as well as the brain, is much more developed than it is in the classes of fish and reptiles. It is known, that the nervous system of those animals which breathe by tracheæ, is less perfect than in those which breathe by lungs; and that in those, where there is no nervous system, that of respiration disappears also. In general, there is a reciprocal relation between the brain and the lungs, especially in birds and the mammalia. The first of these occasions the action of the second, by raising the ribs and favouring the entrance of air into the bronchiæ; the second also keeps up the activity of the first, by means of the red blood which it sends thither. It would be an interesting speculation to inquire into the relation of the nervous system with that of respiration in the class of insects, for as they receive the air by points, which open externally, there seems to be no mechanical action in the process of their breathing, and thus the function appears with them to belong entirely to the organic life. FOOTNOTES: