70 THE LUNGS, BY FRANZ EILHARD SCHULZE. 



passages are given off, which run at right angles, and in a 

 radial direction from the longitudinal axis of the pipes. These 

 are at first straight and simple, but soon branch in an 

 arborescent manner, and indeed usually dichotomously at acute 

 angles, till they terminate in small elongated csecal processes 

 (having a diameter of 0'015 0*009 of a millimeter in the Swan ; 

 of 0-010 0-006 in the Goose ; and of 0'009 O'OOG of a milli- 

 meter in the Pigeon), which, when strongly filled with injection, 

 present numerous irregular lateral dilatations. 



In the membranous wall of all bronchia extending to the 

 surface of the lung, just as in the bronchial walls of the Mam- 

 malia, four separate layers can be distinguished : an external 

 fibrous layer, a muscular layer, though this, perhaps, is not 

 quite continuous, an internal fibrous layer, and an epithelial 

 layer. 



The external fibrous layer consists of fibrous connective 

 tissue, chiefly running in a longitudinal direction, with fine 

 elastic fibres distributed through it. A few thin cartilagi- 

 nous laminse occur at the commencement of the principal air 

 passage, which embrace the tube for about two-thirds of its 

 circumference. Small masses of fat are found scattered here 

 and there in variable quantity, dependent probably on the 

 condition of nutrition of the individual. 



The circular layer of smooth muscular fibres lying on 

 the inner side of the external fibrous layer, is not perfectly 

 continuous, but presents narrow lacuniform spaces ; beneath the 

 cartilages the fibres are altogether absent. The septa projecting 

 from the membranous wall into the lamina of the bronchial 

 tubes contain, especially near the free borders, strong muscular 

 bands, which communicate here and there with those that have 

 been just described by delicate fasciculi. 



The internal fibrous layer is composed of a rather thin layer 

 of connective tissue, the fibres of which run longitudinally, 

 and are intermingled with fine networks of elastic tissue. It 

 presents at some points slight longitudinal ridges, and contri- 

 butes in great measure to form the septa already described as 

 arranged in a plexiform manner. It is invested on its inner 

 free surface by a layer of columnar and ciliated epithelial 

 cells, amongst which are numerous cup cells, and which be- 



