STRUCTURE OP THE LUNGS IN MAN. 377 



contractility of the walls of the smaller bronchi may be excited by 

 chemical, electrical, or mechanical stimuli applied to themselves ; though 

 it is not so readily caused to manifest itself by stimulating the nerves. 

 By the continued influence of galvanism, bronchial tubes of a line ia 

 diameter have been made to contract, until their cavity was nearly oblite- 

 rated ; and it has been found by Volkmann that a similar effect may be 

 produced by galvanising the Par Yagum. Supposing the muscular 

 fibres of the bronchial tubes to contract during expiration, the effect of 

 such contraction would be to diminish both the length and the diameter 

 of the tubes, and thus to force out their contained air. Whether such 

 contraction, alternating with relaxation, takes place automatically, as a 

 part of the ordinary rhythmical movements of respiration, has not yet 

 been clearly made out ; but in its tonic form, it manifests itself strongly 

 in certain diseased conditions, especially in spasmodic Asthma, which 

 appears essentially to consist in a contracted state of the smaller 

 bronchial passages, occasioning an interruption to the passage of air 

 through them. It is interesting to observe, that the contractility of the 

 muscular walls of these tubes has been experimentally found to be 

 greatly diminished by the application of vegetable narcotics, especially 

 stramonium and belladonna, substances which are well-known to have 

 a powerful remedial influence in spasmodic Asthma. 



679. The Lungs themselves appear to be, almost entirely, passive 

 instruments of the Respiratory function. Their contraction when over- 

 distended, and their dilatation after extreme pressure, may be partly due 

 to the elasticity of their structure ; which seems to produce, when acting 

 by itself, a moderately-distended state of the air-cavities. This, too, is 

 the condition that seems most natural to the cavity of the chest ; the 

 fullest dilatation, or the most complete contraction, of which it is capa- 

 ble, being only accomplished by a forcible effort. 



680. The dilatation of the cavity of the chest, which constitutes 

 Inspiration, is accomplished by two sets of movements ; the elevation 

 of the ribs, and the depression of the diaphragm. From the peculiar 

 mode in which the ribs are articulated with the spinal column at one 

 extremity, and from the angle which they make with the cartilages that 

 connect them to the sternum at the other, the act of elevation tends to 

 bring the ribs and the cartilages more into a straight line, and to carry 

 the former to a greater distance from the median plane of the body, 

 whilst the sternum is also thrown forwards. Consequently the eleva- 

 tion of the ribs increases the capacity of the thorax, upwards, forwards, 

 and laterally. The movement is chiefly accomplished by the Scaleni 

 muscles, which draw up the first rib ; and by the Intercostals, which 

 draw the other ribs into nearer proximity with each other, so that the 

 total amount of movement in each rib increases as we pass from above 

 downwards, every one being drawn up by its connexion with the 

 one above it, and being drawn nearer to it by the action of its own 

 intercostals. The elevation of the ribs is further assisted by the Ser- 

 ratus magnus, and by other muscles connected with the spine and the 

 scapula ; and when the respiratory movement is very forcibly performed, 

 the scapula is itself drawn upwards, by the muscles that descend to it 

 from the neck, thus producing an increased elevation of the ribs, and 



