578 OF RESPIRATION. 



. It appears from the researches of M. Bourgery,* that the development of the air-cells 

 continues in the human subject up to the age of thirty, at which time the capacity for re- 

 spiration is the greatest ; it subsequently decreases, especially in persons who suffer from 

 rough, the violence of which expiratory effort frequently causes rupture of the air-cells, 

 and thus gradually produces that emphysematous state of the lungs, which is so common in 

 elderly persons. The power of increasing the volume of air taken in, by a forced inspira- 

 tion, is much less in the old person than in the child, though the average amount of air in- 

 spired may be the same; hence the young person possesses a greater capacity of respiration, 

 as it were, in reserve ; whilst the old man has little, and is, therefore, unfit for great exer- 

 tion. 



b. The Lungs are developed, in the first instance, as diverticula from the cesophageal tube. 

 In the Chick, about the fourth day, a little sacculus is described as shooting forth at its pos- 

 terior and inferior part; and this soon subdivides at its lower part into two; at the same 

 time becoming more separated from the tube, by a constriction around the neck, which soon 

 elongates so as to form the trachea. On the fifth or sixth day, the lung of one side is com- 

 pletely distinct from that of the other, and each 



Fif. 220. * s atla hed to the common pedicle by a pecu- 



liar branch, the future bronchus. The upper 

 portion has much thicker walls than the lower; 

 and these appear to contain a large quantity 

 of vesicular parenchyma, in which the rami- 

 fications of the bronchial tubes subsequently 

 extend themselves. About the tenth or ele- 

 venth day of incubation, these ramifications 

 possess nearly their permanent character and 

 situation. The first trace of the Glottis ap- 

 First appearance of the lungs: a, in a Fowl at pears about the fifth day; it is then a mere 

 four days ; 6, in a Fowl at six days ; c, termina- s lit in the walls of the oesophagus, resembling 

 tion of bronchus in a very young Pig. that by which the ductus pneumaticus of some 



Fishes, opens into the alimentary canal. The 



formation of the cartilaginous rings of the trachea does not commence until after the twelfth 

 day, when they first appear as transverse stria? on the median line of the front only ; they 

 gradually become solid, and extend themselves on either side, until they nearly meet at last 

 on the median line on the back or vertebral side of the tube. 



c. The history of the process in the Human embryo, appears to be very nearly the same. 

 The first appearance of the Lungs takes place about the sixth week, at which time they are 

 simple vesicular prolongations of the cesophageal membrane. Their surface, however, soon 

 becomes studded with numerous little wart-like projections; and these are caused by the 

 formation of corresponding enlargements of their cavity. These enlargements soon become 

 prolonged, and develope corresponding bud-like enlargements from their sides; and in this 

 manner, the form of the organs is gradually changed, a progressive increase in their bulk 

 taking place from above downwards, in consequence of the extension of the bronchial ra- 

 mifications from the single tube at the apex. At the same time, however, a corresponding 

 increase in the amount of the parenchymatous tissue of the lung is taking place ; for this is 

 deposited in all the interstices between the bronchial ramifications, and might be compared 

 with the soil filling up the spaces amongst the roots of a tree. It is in this parenchyma that 

 the pulmonary vessels are distributed ; and the portion of it which extends beyond the ter- 

 minations of the bronchial tubes, seems to act as the nidus for their further extension. It 

 can be easily shown that, up to a late period of the development of the lungs, the dilated 

 terminations of the bronchi constitute the only air-cells (Fig. 220, c) : but, as already men- 

 tioned, the parenchyma subM.-qucntly has additional cavities formed within it. It is a fact 

 of some intercut, as an example of the tendency of certain diseased conditions to produce a 

 r-'turn to forms which are natural to the fretal organism, or which present themselves in other 

 animals, that up to a late period in the development of the Human embryo, the lungs do 

 not nearly fill the cavity of the chest, and the pleura of each side contains a good deal of 

 serous fluid. 



758. The network of vessels on the walls of the air-cells is so minute, that 

 the diameter of the meshes is scarcely so great as that of the capillary ves- 

 sels which inclose them. According to Mr. Addison, the capillaries in the 

 lung of a Toad admit, in iheir natural state, no more than one, or at most two 

 rows of blood-corpuscles ; and the islets of tissue between them are compa- 



* Archives Generates de Medecine, Mars 1843. 



