290 A. 0. V. Ti/nims: 



and traiishifent, and occupy the region between the hist tracheal 

 ring and the tirst bronchial semi-ring. External to it is a resjjira- 

 tory air sac, internal to it is the air space of the trachea, and 

 so the meniln'ane is stretclied between two air chambers. The wall 

 of the membrane is three-layered. The middle layer is of meso- 

 blastic origin, the other two being of hypoblastic origin. 



(b) Membranae internae. — These membranes are the thin inner 

 walls of the bronchi, and thus occupy the region between the fx*ee 

 ends of the bronchial semi-rings. Above they are limited V)y the 

 pessulus, and below, to some extent, by the bronchidesmus. The 

 space between the two membranes is occupied by the subpessular 

 air space. As before, the membrane has three layers, the innermost 

 being bronchial epithelium, much folded in one part, the middle 

 layer of mesoblastic origin, and the outer the epithelium of the air 

 sac. 



(c) Bronchidesmus. — This is a fil)r()us band, passing between the 

 two membranae internae. It is set somewhat obliquely, and is 

 formed by the apposition of the unsymmetrical right and left 

 interbronchial respiratory air sacs. 



(d) Membranae tracheales. — These are the membraneous lateral 

 walls of the tracheal extremity. Embedded in them lie the last five 

 tracheal rings, which are so flattened and reduced that, notwith- 

 standing this cartilaginous framework, the walls are thin, trans- 

 lucent and membraneous. They are separated caudally from the 

 membranae externae by the last tracheal ring. Like the.se latter 

 membranes, they are composed of three layers, and set between t]\3 

 air in the trachea and that in the respiratory air sacs. 



(e) Membrana semilunaris. — This is a thin fold of mucous mem- 

 brane, overlying and projecting from the cranial border of the 

 pessulus. It consists histologically of an inner core of mesoblastic 

 origin, overlaid on each side by the ti-acheal epithelium. It is only 

 slightly marked in the adult, but is of considerable size at one stage 

 in the embryo. 



3. Muscles. — In addition to the sterno-tracheales and tracheo- 

 clavicular muscles, Avhich have relation })oth to the lower and upper 

 parts of the trachea, there are also present a dorsal and a ventral 

 pair of muscles. These latter two pairs correspond to the true 

 syringeal muscles of other lurds. in which, however, they may be 

 of considerable size, and more intimately associated witli the syrinx. 

 These true syringeal muscles are said to be derived from the sterno- 

 hyoid group of muscles, and pass down the trachea to the syrinx 

 (see Svrinx Dictionary of Birds, by Nowton), but in the common 



