Shooting of Cygnus Bewickii. By Thomas Anderson. 337 



where it is bounded by the rough line of attachment of the great 

 pectoral muscle. When examined from the front we find this 

 keel composed of two side plates, separated by about half an inch, 

 and forming with the floor of the sternum proper a cavity or 

 oblong cul de-sac, which runs back for the entire length of the 

 keel. Anteriorily this cavity measures across the base f inch 

 and across the apex | inch (inside measurements), and contains 

 a peculiar fold of the trachea which is prolonged into it to the 

 depth of 2|- inches. 



The upper or internal surface of the sternum upon which the 

 lungs rest presents a convexity along the middle line, which is 

 most marked opposite each extremity of the keel. 



Tracing the trachea downwards from the cervical region we 

 find it there about f inch in diameter, then passing in front of 

 the clavicles, and curving downwards, it takes a perfectly hori- 

 zontal course and lies in the sternal notch. At this point it is 

 slightly larger, and it now enters the cavity of the sternum. 

 Here its upper surface is in approximation with the lower sur- 

 face of the tube as that emerges from the cavity. In fact the 

 two tubes are parallel and the upper or returning one is slightly 

 larger. On removing a triangular portion from the side of the 

 keel and following the course of the trachea, we find it running 

 backwards for 2f inches when it suddenly turns vertically upwards 

 being completely doubled on itself, ascends for an inch, and is 

 again doubled on itself, and now passing horizontally forwards 

 emerges from the cavity in close relation to the entering tube. 

 It now passes underneath the rounded arch of the furcula and 

 ciirving upwards it enters the thorax and divides into the bronchi. 



The cartilaginous rings of the trachea are, in the cervical 

 region, more or less regular. They are slightly deeper in front 

 and these overlap the narrower portion of the posterior rings. 

 They become stronger as they go down, and within the sternum 

 these rings are very strong and here they are of greatest width 

 inferiorly. At the bend there is one large strong ring lying 

 horizontal, and altogether overlapping those on each side ; while 

 at this point the diameter of the trachea, has nearly attained 

 its greatest size, xe inch, but the superior portion of the tube is 

 slightly larger. After passing the arch of the furcula the rings 

 again become smaller and narrower until they enter the thorax. 



Within the sternum the whole structure is stronger than at 

 any other point, and its air-containing capacity is greater ; and 



Ql 



