18 Descriptions of Preparations. 



are pneumatic (i.e. have air cells in the place of the fatty medulla 

 of other bones). In all birds^ though not exclusively in them, at 

 least as regards some of the points to be herewith specified, the 

 following structural arrangements may be noted : — The lower jaw, 

 which is compounded originally of twelve distinct bones, swings 

 upon a moveably articulated os quadratum, which has ordinarily two 

 crura for articulation with the skull, and a largely-developed orbital 

 process. The upper jaw is mainly made up of the premaxillary bone, 

 the maxilla being lost or rudimentary, and its place being taken 

 by the prevomerine bones, which by their outer surface articulate 

 at once with the premaxillary bone and with the quadratojugal 

 rod, and bring thus the upper jaw into connection with the os 

 quadratum. A second chain, also consisting of two bones, viz. the 

 palatine and the pterygoid, serves the same purpose as the quadra- 

 tojugal in connecting the os quadratum with the upper jaw. The 

 squamous never reaches the jugal bone. The occipital always 

 articulates by a single head, which however may, like the occipi- 

 tal condyle in Reptiles, show traces of its primitive composition 

 out of three bones, by being bifid superiorly, with a more or less 

 perfect cup on the atlas. The sclerotic is strengthened by bony 

 plates. The scapular arch is completed inferiorly by the junction 

 of the coracoids to the sternum. Tlie number of the carpals 

 existing in the adult bird as separate bones is, as a rule, two. The 

 tarsals always coalesce, the proximal ones with the tibia above, and 

 the distally-placed with the metatarsus below. The fibula never 

 articulates with the tarso-metatarsus. There are no lumbar verte- 

 brae ; and respiration is mainly effected by the movement of the 

 sternum from above downwards, the vertebral and sternal ribs mov- 

 ing on each other, and on the sternum, all but exclusively in that 

 direction. The length of the cervical region is never less than the 

 height from the ground at which the body is carried by the legs, 

 nor than the length from the root of the neck to the terminal 

 ploughshare-shaped vertebrae which supports the uropygial gland. 

 It depends not, as in mammals, exclusively upon the greater or 

 less length of the bodies of its vertebrae, but also upon the in- 

 creased or diminished number of these vertebrae, which may vary 

 from eleven to twenty- four, being thus in birds the most, as it is 

 in mammals the least, variable number of any of the numbers of 

 the several vertebra series. The dorsal vertebrae, or vertebrae 



