SEC. IV ANATOMY OF BIRDS 199 



bone, and chest ; and of the ajppendkular skeleton, including bones of 

 the limbs, considered as diverging appendages of the trunk. The 

 skeleton is jointed ; bones join either by immovable suture, or by 

 movable artiadation (Lat. articulus, a joint, dimin. of artus, a limb). 

 In free articulations, the opposing surfaces are generally smooth, 

 and lubricated Avith a fluid called synovia. Progressive ossification 

 often causes bones originally distinct to coossifij, that is, to fuse to- 

 gether ; this is termed ankylosis ; bones so melted together are said 

 to be anhjlosed (Gr. ayKvAwo-ts or ay^^yXMcri'i, the stiffening of joints 

 in a bent position). Thus all the bones of a bird's brain -box are 

 ankylosed together, though this box at first consists of many 

 distinct ones • and the determination of such osseous elements or 

 integers in compounded bones is a very important matter, as a clue 

 to their morphological composition. The names of most individual 

 bones, chiefly derived from the old anatomists, are arbitrary and 

 have little scientific signification ; many are fanciful and misleading ; 

 bones named since anatomy passed from the empiric stage, when it 

 was little more than the art of dissecting and describing, however, 

 have as a rule better naming. The shaft of a long bone is its con- 

 tinuity : the enlargements usually found at its extremities are called 

 condyles (Gr. kovSuAos, kondulos, a lump, knot, as of the knuckles). 

 Points where ossification commences in cartilage or membrane are 

 ossijic centres, or osteoses ; valuable clues, usually, to the elements of 

 compound bones. But ossification of individual simple bones may 

 begin in more than one spot, and the several osteoses afterward grow 

 together. This is especially the case with the ends of bones, Avhich 

 often make much progress in ossification before they unite with the 

 shaft or main part ; such caps of bone, as long as they are disunited, 

 are called epiphyses (Gr. kiri, epi, upon ; <j)V(ris, phusis, growth). Pro- 

 trusive parts of bones have the general name of pirocesses, or apophyses 

 (Gr. aiTo, apo, away from, and (pva-cs) ; such have generally no ossific 

 centres, being mere outgrowths. But many parts of a vertebra, 

 which are called "apophyses," have independent ossific centres. 

 The progress of ossification is usually rapid and eflfectual. 



The skeleton of birds is noted for the number and extent of its 

 ankyloses, a great tendency to coossification and condensation of 

 bone-tissue resulting from the energy of the vital activities in this 

 hot-blooded, quick-breathing class of creatures. Birds' bones are 

 remarkably hard and compact. When growing, they are solid and 

 marrowy, but in after life more or fewer of them become hollow and 

 are filled Avith air. This pmeumaticity (Gr. irvivixaTiKo?, pneumatikos, 

 windy) is highly characteristic of the avian skeleton. Air penetrates 

 the skull-bones from the nose and ear-passages, and may permeate 

 all of them. It gains access to the l)ones of the trunk and limbs by 

 means of air-tubes and air-sacs, which connect with the air-passages 



