sion, respiration is discovered," and the same: action is afterwards repeated when the same uneasiness recurs. Dr. Philip thinks the difficulty may be solved by regarding the muscles of inspiration as entirely under the control of the will, and thrown into action by the uneasy sensation which the young animal experiences when it is separated from the mother, and can no longer have the necessary change produced upon the blood by her organs. He supposes the first inspiration to be entirely analogous to the first act of deglutition; the will, in both cases, causing the contraction of certain muscles for the purpose of removing an uneasy sensation." I shall not enter into a formal examination of these hypotheses; they appear to me to be built upon the assumption of principles, which are at least doubtful, if not altogether untenable; and with respect to the explanation offered by Whytt, it labours under the radical defect of all the metaphysical reasoning of the spiritualists, that it confounds the final with the efficient cause, and supposes the agency of an imaginary power, of the existence of which we have no evidence. I think it may be doubted whether we are in possession of any data which will enable us fully to explain the difficulty, but there are some circumstances connected with the mechanical change which the lungs experience at birth, in consequence of the alteration of the position of the animal, that may throw some light upon it. Before birth the Zoonomia, v. i. sect. 16. § 4. 5 Quart. Journ. v. xiv. p. 100. lungs only receive one-third part of the quantity of blood which afterwards circulates through them, and are squeezed up into as small a space as possible from the posture of the foetus," as well as from the larger size of the heart and the liver, and by the thymus gland," so that the cavities of the vesicles and bronchia are nearly obliterated. The arch of the ribs is depressed, and the diaphragm is pushed up into the higher part of the chest, so that its concavity toward the abdomen is greater at this period than it ever afterwards becomes when the animal has once respired." As soon, however, as the position of the animal is changed upon its leaving the uterus, the trunk is extended, and the pressure removed from the thorax and abdomen. The elasticity of the parts being then at liberty to act, the arch of the ribs is raised, and the distance increased between the sternum and the spine, the liver and the other abdominal viscera now fall into their natural position, and permit the diaphragm to assume its ordinary curvature. All • Boerhaave and Haller, in Prælect. t. ii. § 200, cum notis. 7 Harvey de Gener. p. 353; Denman's Midwif. p. 218; Murat, in Dict. Scien. Med, art. "Fœtus," t. xvi. p. 55. Haller, in Boer. Præl. t. v. par. 2. not. ad § 681; and El. Phys. xxix. 4. 39; Denman, p. 158. .161; Blumenbach's Physiol. P. 361; Magendie, Physiol. t. ii. p. 435; Monro's Elem. v. i. p. 576; pl. 10; v. ii. p. 113. 9 Petit, Mem. Acad. pour 1733, p. 6; Senac, ibid. pour 1724, p. 171. See Hunter on the Gravid Uterus, pl. 12, 13, 20; also Sommering, Icon, Embryon. Hum. fig. 18, 19, 20; for the representation of the posture of the fœtus. these changes necessarily increase the capacity of the thorax, and cause the air to rush down the trachea of the animal into the bronchial vesicles, when the blood, meeting with less resistance to its passage through the lungs than through the foramen ovale, the whole of it passes through the pulmonary artery. The organs are thus brought into the state of ordinary expiration, or what I have termed their quiscent condition, when the necessity for inspiration will depend upon the same cause, which renders the alternation of inspiration and expiration essential to the future existence of the animal. According to this view of the subject, the first degree of expansion, which is produced in the lungs of the newlyborn infant, depends merely upon the removal of external pressure, which permits the different parts of the trunk to assume their ordinary position. The farther increase of the size of the chest will depend upon the contraction of the diaphragm, and perhaps, strictly speaking, this contraction should be regarded as constituting the first act of inspiration.' A Besides the hypotheses of Whytt, Haller, and Darwin, which, in consequence either of their supposed merits, or the celebrity of their authors, have acquired some degree of consideration, many others have been formed by physiologists of eminence; as by Borelli, who resolves the question simply into the necessity which now exists for the young animal to perform those functions, which were before exercised by the mother, par. 2. prop. 118; by Pitcairne, Dissert. p. 62; and by Petit, Mem. Acad. pour 1733, p. 6, who refer it to certain general laws, which they suppose to prevail with respect to the action of the muscles and the animal spirits; by Lister, who explains it upon Cause of the Alternations of Inspiration and Expiration. 41 Nearly allied to the question respecting the first commencement of respiration, is the inquiry into the cause of the regular alternations of inspiration and expiration, a subject which has given rise to as many hypotheses and speculations as the former, but being perhaps in itself more difficult of explanation, still remain at least equally involved in obscurity. Some physiologists have considered the necessity for the alternations of respiration to the support of life as a sufficient reason for its existence, thus substituting the principle that the blood, which before birth passed through the umbilical, is now transmitted through the pulmonary ves sels, de Respir. in Exercit. Anat.; by Swammerdam, who conceives that there is in the fœtus a space between the lungs and the thorax, which is filled with an aqueous vapour, which being expelled when the animal first attempts to breathe, enables the air to enter the lungs, De Respir Sect. 2. c. 1; by Boerhaave, Instit. § 691; Hartley, on Man, v. i. p. 95; Buffon, Nat. Hist. v. iii. p. 111; and Blumenbach, Physiol. § 151, who ascribe it to the struggles of the fœtus when it leaves the uterus, by which the muscles generally, and the diaphragm in particular, are thrown into action, and the uneasy sensations which are experienced from diminished temperature, and the contact of surrounding bodies. Dr. Elliotson ascribes it solely to the impression of the cold air upon the surface of the body; Notes to Blumenbach, p. 84. Wrisberg appears to make no distinction between the cause of the first expansion of the chest and the subsequent act of inspiration; De Respir. prima, in Sandifort, Thes. t. iii. p. 258. . 260; Sprengel, Instit. Med. t. i. p. 464; and Parr, Dict. art. "Fœtus," adopt an opinion very nearly similar to that in the text. Semmering, like Borelli, confounds the final with the physical cause; Corp. Hum. Fab. t. vi. § 70. the final for the efficient cause of the action. Others have attributed it to some mechanical effect, depending upon the pressure on the brain or a particular nerve, by the lungs or the diaphragm, at certain stages of the act of respiration. Others again have accounted for it by some speculative principle assumed concerning muscular contraction in general, which they have applied to the organs connected with the chest, among whom we may class Willis, Pitcairne," and Hartley; while others ascribe it to the effect of habit, association, or instinct." 4 It will not be necessary to enter into any minute account of these hypotheses, and still less into any examination or refutation of them, as they appear to have been scarcely maintained except by the individuals who originally proposed them. But it may be proper to examine a little more in detail the opinions which were entertained upon this point by Haller and Whytt, because at one time they acquired considerable reputation, and probably approach somewhat more nearly to a correct view of the subject. Haller sets out with the position, that the passage of the blood through the lungs is impeded during expiration; this produces a reflux of blood into the veins, 2 Borelli, par. 2. p. 117; Bellini, de Urinis, Intr. Lemma 18. 3 Boerhaave, Instit. § 419, 0; Martine, Ed. Med. Ess. v. i. p. 156. 4 Pharm. Rat. par. 2. p. 18. 6 On Man, v. i. ch. 1. prop. 19. s Dissert. par. 4. p. 62. |