OSSEOUS SYSTEM OF AVES. 37 



confluence ; from that of Mammals by being unclosed^, and by the 

 widely perforate acetabulum, fig. 22, i. 



The lars^e size and brittle shell of the eo-jT are the teleoloo^ical 

 conditions of the open pelvis, and the transference of the weight 

 of a horizontal trunk upon a single pair of legs necessitates an 

 extensive grasp of its segments. When the legs require to be 

 pulled far and strongly back, as in diving and cursorial motions, 

 the origins of the requisite muscles are extended far behind the 

 limbs' centre of motion, as in the pelvis of the Grebes, Loons, 

 Guillemots, Ostriches, Emeus ; when the bird slowly stalks, or 

 hops, or climbs, or uses the legs chiefly in grasping and perching, 

 the pelvis is short and broad, especially behind, and its breadth 

 may exceed its length (^Cyclarius guanensis). 



The caudal vertebrje are few, short, not produced into a con- 

 spicuous appendage, the so-called ' tail ' of birds being due to the 

 feathers attached to the terminal vertebrae ; these, in birds of 

 flight, coalesce to the number of two or more, and form a com- 

 pressed vertically extended bone, like a plough-share,^ fig. 23, «, 

 presenting a concave surface to the antecedent centrum ; rising 

 above as a sharp crest, anteriorly perforated by the termination of 

 the neural canal ; expanding below and there perforated by the 

 haemal canal which terminates by one inferior and two lateral ori- 

 fices. This compound bone, ' os en sec de charrue,' supports the 

 coccygeal oil-glands, and gives attachment to the ^ rectrices ' or 

 rudder-quill-feathers, which are disposed fan-wise. In the Wood- 

 peckers the hasmal part extends far in advance of the articular 

 surface of the centrum, and expands into a broad subquadrate 

 plate concave below ; the neural part forms as large a vertical 

 plate ; this relates to the use of the stifl" tail-feathers in climbing. 

 The horizontal developement prevails in the Peacock. Reckoning 

 the terminal bone as one, the common number of caudal vertebra3 

 is nine. The anterior ones have vertically extended transverse 

 processes including di- par- and pleur-apophysial elements ; the 

 neural arch has prezygapophyses, very small postzygapophyses, 

 and a short and thick neural spine. In the third or fourth ver- 

 tebras caudal hjemapophyses appear, increase in length, and in 

 the fifth or sixth inclose a hasmal canal. The transverse pro- 

 cesses in many birds increase in length to the antepenultimate ; 

 in a few (^Ibis, Uria) they gradually shorten to the last ; the 

 caudal centrums are joined by ligament and are procoelian. In 

 the Toucan the joint between the sixth and seventh vertebrae 



> XII. vol. i. p. 208. 



