STRUCTURE AND DEVELOPMENT OF HUMAN LUNG. 



579 



Arrangement of the Capillaries of the air-cells of 

 Ihe Human Lung. 



ratively large ; whilst, if the lung Fi S- 221. 



be congested or inflamed, five or 

 six rows of corpuscles are seen in 

 the vessels ; and the islets of tissue 

 are almost entirely obliterated. 

 The diameter of the Human air- 

 cells is about twenty times greater 

 than that of the capillaries which 

 are distributed upon their parietes ; 

 varying (according to the measure- 

 ment of Weber) from the 1 -200th 

 to the l-70th of an inch. It has 

 been calculated by M.Rochoux, that 

 as many as 17,790 air-cells are 

 grouped around each terminal bron- 

 chus ; and that their total number 

 amounts to no less than 600 mil- 

 lions. 



759. The fibrous coat of the bronchial tubes possesses a considerable amount 

 of contractility, which can scarcely be regarded as otherwise than muscular. 

 From the experiments of Dr. C. B. Williams,* it appears that all the air- 

 tubes are endowed with a considerable amount of contractility, which may 

 be excited by electrical, chemical, or mechanical stimuli, applied to them- 

 selves ; but this is not so readily excitable through their nerves, although the 

 experiments of Volkmann and Longet have clearly shown the possibility of 

 thus calling it into action ( 410). This contractility resembles that of the 

 intestines or arteries, more than that of the voluntary muscles or heart; the 

 contraction and relaxation being more gradual than that of the latter, though 

 less tardy than that of the former. It is chiefly manifested in the smaller 

 bronchial tubes ; since, in the trachea and the larger bronchi, the cartilaginous 

 rings prevent any decided diminution in the calibre of the tube. Wedemeyer 

 did not succeed in producing any distinct contraction of the fibres of the tra- 

 chea and larger bronchi ; but he states that tubes of less than a line in dia- 

 meter could be perceived to contract gradually under the stimulus of galva- 

 nism, until their cavity was nearly obliterated. It is remarked by Dr. Williams, 

 that the contractility of the bronchial muscles is soon exhausted by the action 

 of a stimulus ; but that it may in some degree be restored by rest, even when 

 the lung is removed from the body. When the stimulation is long continued, 

 however, as by intense irritation of the mucous membrane during life, the 

 contractile tissue passes into a state which resembles that of the tonic con- 

 traction of muscular fibre ( 593). The contractility is greatly affected by 

 the mode of death, and is remarkably diminished by the action of vegetable 

 narcotics, particularly stramonium and belladonna ; whilst itseems to be scarcely 

 at all affected by hydrocyanic acid. These facts are very important, as 

 throwing light upon certain diseased conditions. It has long been suspected, 

 that the dyspnoea of Spasmodic Asthma depends upon a constricted state of 

 the smaller bronchial tubes, excited through the nervous system, frequently 

 by a stimulating cause at some distance ; and there can now be little doubt 

 that this is the case. The peculiar influence of stramonium and belladonna, 

 in diminishing the contractility of these fibres, harmonizes remarkably with 

 the well-known fact of the relief frequently afforded by them in this distress- 

 ing malady. 



760. The Lungs themselves are to be regarded :>* quite passive in the 



Athenceum Report of the Meeting of the British Association, 1S40, p. 802. 



