PHYSICAL PROPERTIES OF THE LUNGS. 165 



simple lung of the newt, there are also numerous nerve-cells disposed along the course of the 

 intra-pulmonary nerves. Some of these fibres terminate in the uniform layer of non-striped 

 muscle which forms part of the pulmonary wall in the frog and newt, and others end in the mus- 

 cular coat of the pulmonary blood-vessels (Stirling). The functions of these ganglia are unknown, 

 but they may be compared to the nerve-plexuses existing in the walls of the digestive tract.] 



The Function of the non-striped muscle of the entire bronchial system seems to 

 be to offer a sufficient amount of resistance to increased pressure within the air- 

 passages ; as in forced expiration, speaking, singing, blowing, &c. The vagus is the 

 motor nerve for these fibres, and according to Longet, the " lung-tonus " during 

 increased tension depends upon these muscles. 



[Effect of Nerves. By connecting the interior of a small bronchus with an oncograph ( 103) 

 in curarised dogs (the thorax being opened), Graham Brown and Roy found that section of one 

 vagus causes a marked expansion of the bronchi of the corresponding lung, while stimulation 

 of the peripheral end of a divided vagus causes a powerful contraction of the bronchi of both 

 lungs. Stimulation of the central end of one vagus, the other being intact, also causes a con- 

 traction (feebler) under the same circumstances. Especially in etherised dogs, expansion and 

 not contraction results. If both vagi be divided, no effect is produced by stimulation of the 

 central end of either vagus. It seems plain that the vagi contain centripetal or afferent fibres, 

 which can cause both expansion and contraction of the bronchi. Asphyxia causes contraction 

 provided the vagi are intact, but none if they are divided, although in etherised dogs expansion 

 frequently occurs, while stimulation of the central end of other sensory nerves has very rarely 

 any, or, if any, but a slight, effect on the calibre of the bronchi, so that in the dog the only 

 connection between the cerebro-spinal centres and the bronchi is through the vagi.] 



Pathological. Stimulation of the smooth muscles, whereby a spasmodic narrowing of the 

 smaller bronchi is produced, may excite asthmatic attacks. If the expiratory blast be interfered 

 with, acute emphysema may take place (Biemier). 



Chemistry. In addition to connective, elastic, and muscular tissue, the lungs contain lecithin, 

 inosit, uric acid (taurin and leucin in the ox), guanin, xanthin (?), hypoxanthin (dog) soda, 

 potash, magnesium, oxide of iron, much phosphoric acid, also chlorine, sulphuric, and silicic 

 acids in diabetes sugar occurs in purulent infiltration glycogen and sugar in renal degenera- 

 tion urea, oxalic acid, and ammonia salts ; and in diseases where decomposition takes place, 

 leucin and ty rosin. 



[Physical Properties of the Lungs. The lungs, in virtue of the large amount 

 of elastic tissue which they contain, are endowed with elasticity ; and when the 

 chest is opened they collapse. If a cannula with a small lateral opening be tied 

 into the trachea of a rabbit's or sheep's lungs, the lungs may be inflated with a 

 pair of bellows, or elastic pump. After the artificial inflation, the lungs, owing 

 to their elasticity, collapse and expel the greater part of the air. As much air 

 remains within the light spongy tissue of the lungs, even after they are removed 

 from the body, a healthy lung floats in water. If the air-cells are filled with 

 pathological fluids or blood, as in certain diseased conditions of the lung 

 (pneumonia), then the lungs or parts thereof may sink in water. The lungs of the 

 foetus, before respiration has taken place, sink in water, but after respiration has 

 been thoroughly established in the child, the lungs float. Hence, this hydrostatic 

 test is largely used in medico-legal cases, as a test of the child's having breathed. 

 If a healthy lung be squeezed between the fingers, it emits a peculiar and character- 

 istic fine crackling sound, owing to the air within the air-cells. A similar sound is 

 heard on cutting the vesicular tissue of the lung. The colour of the lungs varies 

 much ; in a young child it is rose-pink, but afterwards it becomes darker, especially 

 in persons living in towns or a smoky atmosphere, owing to the deposition of 

 granules of carbon. In coal-miners the lungs may become quite black.] 



[Excision of the Lung. Dogs recover after the excision of one entire lung, and they even sur- 

 vive the removal of portions of lung infected with tubercle (Biondi).] 



107. MECHANISM OF RESPIRATION. The mechanism of respiration 

 consists in an alternate dilatation and contraction of the chest. The dilatation 

 is called inspiration, the contraction expiration. As the whole external surfaces 

 of both elastic lungs are applied directly, and in an air-tight manner, by their 

 smooth moist pleural investment to the inner wall of the chest, which is covered by 



