STRUCTURE OF BIRDS. 519 



form wings, serviceable in nearly all cases for flight. There 

 are never more than three digits in the hand, two of them 

 usually much reduced, and none of them bearing claws 

 (with rare exceptions); nor more than two separate carpal 

 bones in adult recent birds ; nor any separate interclavicle ;. 

 the clavicles are normally complete, and coalesce to form a 

 " merry - thought. " The sternum is large, and usually 

 keeled (the only exception among recent forms being the 

 struthious birds) ; it ossifies from two to five or more centres,, 

 and the ribs are attached to its sides. The skull articulates 

 with the spinal column by a single median convex condyle, 

 develoijed in connection with a large ossified basi-occipital. 

 The lower jaw consists of several pieces, articulated by a 

 quadrate bone to the skull, and in all recent birds both jaws 

 are tootlaless and encased in a horny beak. The bodies of 

 at least some of the vertebriB of recent birds have sub-cyclin- 

 drical, articular faces ; when these faces are spheroidal, they 

 are opisthocoelian, but some fossil forms are amphiccelian. 

 The proper sacral vertebras have no expanded ribs abutting- 

 against the ilia. The ilia are greatly prolonged forwards ;. 

 the acetabulum is a ring, not a cup ; the ischia and pubes- 

 are prolonged backwards ; there is no ischial symphysis ; 

 there may be a prepubis ; a process of the astragalus early 

 anchyloses with the tibia. Tlie incomplete fibula does not 

 reach the ankle-joint ; there are not more than four digits,, 

 the normal numbers of phalanges of which are 3, 3, 4, 5. 

 The 1st metatarsal is incomplete above ; the 2d, 3d and 4tb 

 anchylose together, and with the distal tarsal bone unite to- 

 form a tarso-metatarsus.* The heart is completely four-cham- 

 bered ; there is but one aortic arch (the right), and but one- 

 pulmonic -trunk from the right ventricle ; the blood is red 

 and hot. The large lungs are not free in the cavity of th& 

 thorax, but fixed and moulded to the walls of that cavity .; 

 and in all recent birds the larger air-passages of the lungs, 

 terminate in air-sacs. More or fewer of the bones are 

 usually hollow, and permeable to air from tlie lungs. There 

 is at most a rudimentary diaphragm. The eggs are very large, 

 in consequence of a copious supply of albuminous substance, 

 in the form of yolk and white, and are enclosed in a hard 

 * This structure is diagnostic of birds. 



