SEC. IV 



ANATOMY OF BIRDS 



arch. The hindmost cranial segment, the occipital bone, resembles 

 a vertebra in many physical characters, and even in mode of 

 development. But if the serial homology of the skull with the 

 back-l)one be real and true, it is so obscured by the extraordinary 

 modifications to which the vertebral elements have been subjected 

 that the fact of such homology cannot Ije demonsti'ated ; and to 

 interpret the skull as something superimposed upon and morpho- 

 logically difterent from the spinal column, is perfectly warranted if 

 not required by the known facts of its constructive development. 

 This is the view taken by the rulers of to-day's science. As already 



Fig. 63.— Skull of a duck (Clangula islandica), nat. size ; Dr. R. W. Shufeldt, U.S.A. a, 

 premaxillary bone ; h, partly o.ssified internasal septum ; b', pervious part of nostril ; c, end of 

 premaxillary, perforated for numerous branches of second division of the fifth cranial nerve ; d, 

 dentary bone of under mandible ; e, groove for nerves, etc.; /, a vacuity between dentary and 

 other pieces of the mandible ; g, articular surface ; h, recurved " angle of the ,]a.w ; " i, occipital 

 protuberance ; j, vacuity in supraoccipital bone ; k, muscular impression on back of skull ; I is 

 over the black ear-cavity ; m, postfrontal process ; n, quadrate bone ; o, pterygoid ; p, pala- 

 tine ; q, quadratojugal ; r, jugal ; s, maxillary ; t, fronto-parietal dome of the brain-cavity ; «, 

 the lacrymal bone, immense in a duck, nearly completing rim of the orbit by approaching ?» ; 

 V, vomer ; v\ supraorbital depression for the nasal gland (see p. 231) ; z, cranio-facial hinge ; 

 y, optic foramen ; z, etc., interorbital vacuities. 



said (p. 202), the relation between cranial and vertebral parts is 

 rather the analogy of adaptive modification than a true homology 

 of structure. 



Before proceeding to describe the mature skull, it will be best 

 to consider its mode of development. In this I shall closely follow 

 Parker, often using the words of that master, and illustrating the 

 early stages of the embryo with figures borrowed from the same 

 safe source. In the fewest words possible, I wish to convey an 

 idea of the embryonic skull up to Parker's " third stage," at which 

 it begins to ossify. Here, however, I will first insert a figure (Fig. 

 62), kindly drawn for me by Dr. R. W. Shufeldt, of the U.S. Army, 

 which shows most of the cranial bones, and will give the student a 



