SKELETON OF THE TRUNK. 13 



deed almost always observed. The number of cervical verte- 

 bras varies much ; most generally there are twelve or fifteen ; 

 but sometimes we find a much larger number, and at others, not 

 so many ; the Swan has twenty-three, and the Sparrow only 

 nine. These bones are always very moveable on each other, and 

 from the disposition of their articular surfaces, the neck may be 

 bent like the letter S, and, consequently, be elongated or shortened 

 accordingly as the curves are diminished or increased. 



9. The bony frame of the trunk is very solid ; in birds that 

 fly, (and with the exception of a very few, they all possess this 

 faculty,) the vertebrae of the back, which necessarily support the 

 ribs, and consequently afford a point of support for the wings, 

 are entirely immoveable and are frequently anchylosed, that is, 

 soldered together ; the lumbar and sacral vertebras are all united 

 into one bone, having the same uses as the sacrum in the mam- 

 malia : finally, the coccygian vertebrae are small and moveable ; 

 the last one, which sustains the large tail feathers, is ordinarily 

 larger than the others and marked by a projecting spine or crest. 



1 0. The ribs of birds also possess some peculiarities of struc- 

 ture which tend to increase the strength of the thorax ; but the 

 most remarkable part of the bony frame of this division of the 

 body is the sternum, which, affording points of origin for the 

 chief muscles of flight, becomes very much developed, and con- 

 stitues a broad shield or breast-plate, which extends far back 

 over the abdomen, and almost always presents a sort of very 

 prominent and longitudinal crest or keel, called brisket. [Plate 

 If Jig. 2.) It is remarked that this shield is most developed and 

 most completely ossified in those birds that fly best. 



1 1. The bones of the shoulders are disposed in a manner most 

 favourable for the power of the wings ; they are three in number, 

 namely; a Scapula, a Clavicle^ and a Coracoid Bone. The 

 Scapula is much elongated ; the Clavicle is anchylosed with that 

 of the opposite side, so as to form a bone resembling in shape, 

 the letter V, the point of which rests against the Sternum ; the 

 Coracoid bone, or posterior clavicle, is a sort of second clavicle, 

 which, in the mammalia, is rudimentary and confounded with 

 the Scapula, but here becomes very strong, consituting a buttress 

 placed between the articulation of the shoulder and the sternum. 



9. What is remarked of the skeleton of the trunk ? Are the vertebraB of 

 the spinal column moveable on each other? 



10. What is remarkable of the sternum of birds ? What is the brisket? 

 What renders it necessary that the sternum should be large ? 



11. How many bones belong to the shoulder? What are they? What 

 is the form and situation of the clavicle ? What is the coracoid bone ? 

 What is its situation ? What is the advantage derived from the double 

 clavicles in birds ? 



