RESPIRATION. 



337 



When the external air is admitted freely into 

 the sac of the pleura, by an opening in the 

 parietes of the thorax sufficiently large to 

 permit the air to pass through it in greater 

 quantities than it can enter the lungs by the 

 trachea, the lung collapses rapidly and is 

 compressed against the spine; and if this 

 take place on both sides of the chest, the 

 respiratory process is arrested, and the indi- 

 vidual dies, as from suffocation. When the 

 lungs lose their elasticity, and the air-cells 

 become dilated and their septa partially broken 

 down, as in emphysema, the respiratory mem- 

 brane is not only diminished in extent, but 

 expiration is more difficult, and when the 

 chest is laid open after death, the lungs col- 

 iapse imperfectly or not at all. It is evident 

 that still more serious evils must follow inter- 

 lobular emphysema, or effusion of air into the 

 cellular tissue surrounding the smaller lobes 

 of the lungs, if this occurs to a considerable 

 extent. 



Though the trachea, the bronchii, and even 

 the smaller bronchial tubes are provided with 

 distinct muscular fibres which can be thrown 

 into contraction by direct excitation, and 

 even, according to some experimenters, by 

 excitation of their nerves, yet the notion 

 entertained by many of the older, and even 

 by some modern physiologists, that the lungs 

 have an active power of contraction and 

 dilatation synchronous with and aiding the 

 movements of inspiration and expiration, is 

 undoubtedly untenable. These muscular fibres 

 of the bronchial tubes are endowed with that 

 kind of contractility termed simple contracti- 

 lity, which manifests itself by more slow and 

 prolonged contractions and relaxations than 

 that of the voluntary muscles and the heart.* 

 The possession of this property of simple 

 contractility unfits these muscular fibres from 

 acting' simultaneously with the muscles of 

 respiration moving the thorax, but fits them 

 for effecting these changes on the capacity of 

 the air-tubes, which may aid in the expulsion 

 of substances from their interior, as in cough- 

 ing. The movements of the cilia placed on 

 the inner surface of the respiratory organs, 

 can assist little, if at all, in renewing the at- 

 mospheric air in the lungs. The passage of 

 the air into and from the lung, has an im- 

 portant effect upon the muscular respiratory 

 movements. When a lung, or a considerable 

 portion of it, is prevented from expanding 

 by disease or any other cause, the pressure 

 of the air on the inner surface of that por- 

 tion of the chest covering the unexpansible 



sheep, and large dogs, the resiliency of the lungs 

 was found to be balanced by a column, of water 

 varying in height from one foot to a foot and a half, 

 and in rabbits and cats by a column of water vary- 

 ing in height from six to ten inches." Vide also 

 Observations by M. P. Berard on the Effects of the 

 Elasticity of the Lungs, in Archives Gene'r. de Me'- 

 decine, torn. xxii. p. 180. 1830. 



* Vide experiments of Wedemeyer (Untersuch- 

 ungen iiber den Kreislauf, p. 70), 'and of Dr. C. J. 

 B. Williams (Transact, of British Scient. Assoc. for 

 1840, p. 411), upon the contractility of the bron- 

 chial tubes. 



lung is not now exercised during its dilata- 

 tion ; in other words, this portion of the 

 chest in expanding must do so in opposition 

 to the whole of the atmospheric pressure on 

 its outer surface, amounting to 15 pounds on 

 the square inch. This pressure appears to be 

 too great for the muscles of inspiration, acting 

 upon that part of the chest, to overcome, fop 

 the ribs are there motionless or nearly so, 

 and if the lung is in a state of collapse, the 

 walls of the thorax covering it fall in. 



The muscular movements of inspiration 

 and expiration are, in the natural and healthy 

 state of the body, performed without the in- 

 tervention of volition, and even without our 

 consciousness, and belong to the class of 

 movements which have lately received the 

 appellation of excito-motary. When, how- 

 ever, the free aeration of the blood in the 

 lungs is impeded, a sensation, urgent and 

 imperious in its demands, is felt, which in 

 our language is somewhat clumsily designated 

 " the sensation of the want of fresh air in the 

 lungs," and more elegantly in French, le 

 besoin de respirer. These respiratory move- 

 ments, therefore, depend upon the transmis- 

 sion inwards of certain excitations along 

 afferent nerves to the central organs of the 

 nervous system, whence a motive influence is 

 sent outwards along the motor or efferent 

 nerves, distributed in the muscles to be 

 moved. One of the principal exciter or af- 

 ferent nerves of respiration is the par vagum j 

 and the medulla oblongata is the portion of 

 the central organs of the nervous, system to 

 which all the excitations of the nervous 

 system capable of producing a respiratory 

 muscular movement must be brought. The 

 motor or efferent nerves that convey outwards 

 from the medulla oblongata the motive in- 

 fluence which stimulates the muscles of respi- 

 ration to contract are the phrenic, and part of 

 the anterior roots of the dorsal and lumbar 

 spinal nerves, the recurrent lanyngeal, the 

 portio dura, the spinal accessory, and some 

 branches of the cervical and upper part of the 

 axillary plexus besides the phrenic, especially 

 the branch distributed in the serrattis magnus 

 muscle, termed by Sir Charles Bell the ex- 

 ternal respiratory. Some of these efferent 

 nerves, like the muscles in which the}' are 

 distributed, are habitually engaged in carry- 

 ing on the respiratory muscular movements, 

 while others aid these only when the respira- 

 tion requires to be carried on mere vigorously 

 than usual. 



We have already pointed out the extent 

 to which the nervus vagus acts in conveying 

 to the central organs of the nervous system 

 those impressions that excite the besoin de 

 respirer and the muscular movements of in- 

 spiration. (Vide art. PAR VAGUM.) 



It is impossible to determine whether or 

 not the pulmonary ganglionic nerves can con- 

 vey inwards to the central organs of the 

 nervous system impressions capable of excit- 

 ing the respiratory muscular movements ; but 

 that impressions capable of exciting such 

 movements to a certain extent may be re- 



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