396 ON RESPIRATION. 



development of the lungs, the dilated terminations of the bronchi constitute the 

 only air-cells (Fig. 97, c) : but, as already mentioned, the parenchyma subse- 

 quently has additional cavities formed within it. It is a fact of some interest, 

 as an example of the tendency of certain diseased conditions to produce a return 

 to forms which are natural to the foetal organism, or which present themselves 

 in other animals, that up to a late period in the development of the human em- 

 bryo, the lungs do not nearly fill the cavity of the chest, and the pleura of each 

 side contains a good deal of serous fluid. 



527. The network of vessels on the walls of the air-cells is described by 

 Reissessen as so minute, that the diameter of the meshes is scarcely so great 

 as that of the capillary vessels which form it. According to Mr. Addison, the 

 capillaries in the lung of a Toad admit, in their natural state, no more than 

 one, or at most two rows of blood-corpuscles ; and the islets of tissue between 

 them are comparatively large : whilst, if the lung 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 pulmonary vesicles is 

 about twenty times greater than that of the capillaries which are distributed 

 upon their parietes ; varying (according to the measurement of Weber) from 

 the -nf th to the g\th of an inch. There is no evidence that the alteration in 

 the size of the air-cells, which takes place during the respiratory process, is 

 due to any other cause than the simple elasticity of their walls ; but the bron- 

 chial tubes certainly possess 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 con- 

 siderable amount of irritability, which may be excited by electrical, chemical, 

 or mechanical stimuli, applied to themselves, but not readily (if at all) excitable 

 through their nerves. 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 trachea and larger 

 bronchi ; but he states that tubes of less than a line in diameter could be per- 

 ceived to contract gradually under the stimulus of galvanism, until their cavity 

 was nearly obliterated. It is remarked by Dr. Williams, that the irritability 

 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 contraction of muscular 

 fibre ( 390). 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 it seems to be scarcely at all affected by 

 hydrocyanic acid. These facts are very important, as throwing light upon cer- 

 tain diseased conditions. It has long been suspected, that the dyspnoea of Spas- 

 modic 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. That they 

 should not be readily excited to contraction by a galvanic stimulus applied to 

 their nerves, is no valid argument against this view ; as it was long held that 

 the muscular coat of the alimentary canal also was completely removed from 

 nervous influence, which is now well known to be not by any means the case. 



* Athenaeum Report of the Meeting of the British Association, 1840, p. 802. 



