ELEMENTS OF PHYSIOLOGY. CHAP. VII. OF RESPIRATION. NEXT to the circulation of the blood, the function which is the most essential to life, at least in the higher orders of animals, is respiration. Respiration consists in the alternate reception and emission of air into and out of the lungs, at the same time that the blood is transmitted through a set of vessels so situated, as to enable the air to act upon it, and tọ produce that change in its nature and properties, which fits it for the support of life.' I shall arrange I shall 'It is scarcely necessary to remark, that the above description applies only to the higher orders of animals, the mammalia, birds, and amphibia. In fishes, the process which is equivalent to respiration is performed by the branchia or gills, which are placed in a passage communicating with the fauces, and terminating on the surface of the body, through which a portion of the water received into the mouth is forcibly propelled. It is thus brought into close approximation with the blood which circulates through their fringed extremities, where it receives VOL. II. B my remarks upon this subject under three heads; first, the mechanism of respiration; second, its direct effects; and third, its remote effects on the living system. its appropriate change from the air which the water retains in solution. The mode in which the respiration of fishes is effected, which was imperfectly understood by Boyle, Works, v. i. p. 109, was correctly described by Mayow, Tract. i. c. 15 p. 259; although like many of his discoveries, it appears to have been forgotten, when it was again pointed out by Priestley; On Air, v. iii. p. 342, v. v. p. 136. et seq., of the 1st sec. and v. iii. p. 382 of the 2d series. The respiration of fishes has been since examined by Carradori; Ann. de Chim. t. xxix. p. 171, 2; and by Cuvier; Leçons d'Anatomie Comp. t. iv, p. 305, 6. We have also a very interesting and elaborate set of experiments on this subject by Humboldt and Provençal; Mem. d'Arcueil, t. ii. p. 359. et seq, to which I shall have occasion to refer more particularly in a subsequent section of this chapter. We have also some very valuable experiments by Dr. Edwards, particularly on the relation which the respiration of fishes bears to that of animals that are furnished with lungs ; De l'Influence des Agens, &c. par. ii. ch. 3. M. Dumeril has given an elaborate dissertation on the mechanism of the respiratory organs of fishes; Nicholson's Journ. v. xxviii. p. 350. et seq. translated from Mag. Encyc. Nov. 1807, p. 35. In many of the invertebrated animals the respiratory organs consist merely of a number of tubes or pores, called tracheæ, provided with open mouths, which simply admit the air to enter into them, while there are numerous tribes in which no distinct apparatus can be detected: it appears, however, that in all cases the animal produces the same kind of change upon the air. Scheele noticed the effect of a leach in abstracting the oxygen from water; On Air and Fire, p. 167. Vauquelin found that insects and snails consume oxygen and generate carbonic acid; Ann. de Chim. t. xii. p. 273. et seq. Spallanzani repeated and diversified Vauque § 1. Mechanism of Respiration. The principal organs of respiration in man are the trachea with its ramifications, the pulmonary system of blood-vessels, the lungs, and the diaphragm. The first constitutes the passage by which the air is conveyed into its appropriate receptacles; the sanguiferous vessels are the apparatus by which the blood is carried through the lungs in such a manner, as to enable it to receive the influence of the air; these two sets of parts, with the connecting membranous matter, compose the lungs, while the diaphragm is the principal agent in the alternate enlargement and contraction of the cavity of the thorax. The trachea is a tube composed of cartilaginous rings, united together by elastic ligaments, and furnished with muscular fibres, which commences in the fauces and descends into the thorax. It first divides into two branches, which pass respectively into the two lungs; here it is sub lin's experiments, and obtained the same results with respect to the oxygen and carbonic acid, but he conceived that they also consumed nitrogen; Mem. sur la Respir. p. 184. et alibi. He also details a variety of experiments, in which animals that possessed no distinct organs of respiration deoxidated the air in the same manner with those that have lungs; Mem. p. 258, 301. et alibi. We may infer that in all these cases the same kind of change is effected on the blood or other analogous fluid, for it is this change which is to be regarded as the ultimate object and essence of the function; Magendie, Phys. t. ii. p. 261. For a judicious summary of the various experiments that have been performed on the lower classes of animals, the reader is referred to the third chapter of Mr. Ellis's "Inquiry," and the additions to c. 3, in the "Further Inquiries." my remarks upon this subject under three heads; first, the mechanism of respiration; second, its direct effects; and third, its remote effects on the living system. its appropriate change from the air which the water retains in solution. The mode in which the respiration of fishes is effected, which was imperfectly understood by Boyle, Works, v. i. p. 109, was correctly described by Mayow, Tract. i. c. 15 p. 259; although like many of his discoveries, it appears to have been forgotten, when it was again pointed out by Priestley; On Air, v. iii. p. 342, v. v. p. 136. et seq., of the 1st sec. and v. iii. p. 382 of the 2d series. The respiration of fishes has been since examined by Carradori; Ann. de Chim. t. xxix. p. 171, 2; and by Cuvier; Leçons d'Anatomie Comp. t. iv, p. 305, 6. We have also a very interesting and elaborate set of experiments on this subject by Humboldt and Provençal ; Mem. d'Arcueil, t. ii. p. 359. et seq., to which I shall have occasion to refer more particularly in a subsequent section of this chapter. We have also some very valuable experiments by Dr. Edwards, particularly on the relation which the respiration of fishes bears to that of animals that are furnished with lungs; De l'Influence des Agens, &c. par. ii. ch. 3. M. Dumeril has given an elaborate dissertation on the mechanism of the respiratory organs of fishes; Nicholson's Journ. v. xxviii. p. 350. et seq. translated from Mag. Encye. Nov. 1807, p. 35. In many of the invertebrated animals the respiratory organs consist merely of a number of tubes or pores, called tracheæ, provided with open mouths, which simply admit the air to enter into them, while there are numerous tribes in which no distinct apparatus can be detected: it appears, however, that in all cases the animal produces the same kind of change upon the air. Scheele noticed the effect of a leach in abstracting the oxygen from water; On Air and Fire, p. 167. Vauquelin found that insects and snails consume oxygen and generate carbonic acid; Ann. de Chim. t. xii. p. 273. et seq. Spallanzani repeated and diversified Vauque |