52 THE LUNGS, BY FRANZ EILHARD SCHULZE. 



in newly born Mammals, but subsequently, from mutual pressure, 

 become polyhedral. They are most shallow, whilst they are at 

 the same time separated by tolerably thick septa, in the first 

 portions of each system of alveolar passages that have been 

 already described as constituting the immediate continuation 

 of the smallest bronchial branches; on the other hand, they are 

 deepest, and only separated by thin septa, in the terminal pro- 

 cesses or infundibula, and pre-eminently so at their fundus. 

 The length and breadth of alveoli of moderate depth are about 

 equal. In those parts of the system of alveolar passages in 

 which the diameter of the alveoli is least subject to variation 

 the breadth constantly increases with age, whilst in advanced 

 age the depth diminishes. The diameter of the alveoli in Man 

 during the middle period of life amounts upon the average to 

 0*15 of a millimeter. Soon after birth it varies from O'lO to 

 0'08 of a millimeter, in moderately advanced age (Woman, 

 sixty years of age), the breadth is 0'25 G'4, and the depth 

 O'l 0'2 of a millimeter. The alveoli of the larger domestic 

 animals present very similar dimensions, but they are much 

 less in the smaller species. Thus I found the breadth of the 

 alveoli in Rabbits to measure on the average 0'05; in the Rat, 

 0'04; and in the Mouse and Bat, 0'03 002 of a millimeter. 

 The four to six-sided orifices of the alveoli, with their rounded 

 angles, are generally rather smaller than the alveoli themselves, 

 partly on account of the radial arrangement of the air cells, and 

 partly on account of a slight thickening of the free borders of 

 the alveolar septa. 



In the description of the structural characters of the several 

 parts it will be advantageous to separate that of the bronchia 

 which are destined only for the conduction of air, from that 

 of the parenchyma of the lung, consisting essentially of the 

 alveoli, which is subservient to the interchange of gases 

 between the air and the blood. 



The bronchia of Mammals present remarkably similar histo- 

 logical features, the tubes of equal diameter having in general 

 an analogous or identical structure. On the other hand, con- 

 siderable differences exist coincidently with variations in size. 



In the larger bronchia, down to those that do not exceed 

 one millimeter in diameter, which may be first described, four 



