COMMON PIGEON. 55 



not certain. It appears early in development as a solid outgrowth of cells from 

 the dorsal wall of the proctodaeum ( = cloaca) before the rectal aperture opens into 

 it. The central cells atrophy and thus form the cavity of the organ. In the 

 adult, according to Stieda, the walls consist of a fibrous outer coat with internal 

 prolongations which form the axes of the primary and secondary longitudinal folds 

 which project internally ; and an internal epithelium which has several layers of cells, 

 the superficial columnar, the deep angular, separated from the fibrous coat by a 

 membrana propria. In addition there are the ' follicles ' so-called, which are im- 

 bedded in the longitudinal folds. Each follicle consists of a central mass of 

 minute rounded nucleated cells, continuous at the apex of the follicle with the 

 deep layer of the epithelium, and probably never cut off from it ; of a membrana 

 propria continuous with that of the epithelium ; and of a surrounding investment 

 of adenoid or reticular tissue with numerous capillaries developed from the same 

 embryonic cells as the fibrous coat. 



The respiratory system has the usual structure seen in birds. The syrinx 

 or lower larynx is simple. The last 3-4 rings of the trachea send towards one 

 another on the ventral surface processes which do not fuse. The two last rings 

 are widely separated dorsally, but they are joined inter se by a cartilaginous rod. 

 The bronchial rings are only half-rings, i. e. are incomplete on the inner surface 

 of the bronchus. The free ends of the two first rings meet dorsally and ventrally, 

 and together with the modified tracheal rings form the tympanum. A thin membrane 

 the membrana tympaniformis interna forms the inner wall of the anterior part 

 of each bronchus where the ends of the half-rings are wide apart. Inside the 

 tympanum the mucous membrane is thickened, and there are two cushion-like 

 projections, one on each side. A fold, the membrana semilunaris, projects forwards 

 between the cushions from the point of bifurcation of the trachea. It is supported 

 by the pessulus, a rod of cartilage, sometimes ossified. The Pigeon has but one pair 

 of syringeal muscles the tracheales laterales. The syrinx is least developed in the 

 Cathartidae^ or American Vultures. It is a distinctive avian structure, and is present 

 in the Ratitae, where it is often said to be absent, and it is best developed in Rhea. 

 The trachea is tied to the sternum by a pair of muscles the sterno-tracheales. 



The structure of the lungs, bronchi and air-sacs, is very similar in all birds. 

 The main bronchus enters the lung, gives off above the spot where the pulmonary 

 artery crosses it four eparterial bronchi and below that spot nine hyparterial. The 

 ultimate branches of the bronchi are long tubes closely packed. Their inner 

 walls carry projecting circular fibrous septa connected by longitudinal septa. The 

 respiratory capillaries are distributed on these septa. An epithelium lining these 

 tubes has not been detected. Other structures to be carefully noted are the costo- 

 pleural muscles, small muscular bundles which spring from the junction of the 

 vertebral and sternal ribs and spread out like a fan in an aponeurosis (i. e. ten- 

 dinous expansion) which lies on the ventral aspect of the lung between the pleura 

 and the air-sacs. The air-sacs appended to certain bronchi are nine in number 

 and are as follows, an interclavicular sac lying in front of the trachea and formed 

 by the fusion of two sub-bronchial sacs, the bronchial apertures of which lie just 

 anterior to the spot where the bronchi enter the lungs ; two prae-bronchial sacs 

 lying one dorsal to each lobe of the interclfcficular sac, with apertures at the an- 

 terior border of the lung : two anterior intermediate or thoracic sacs with aper- 



