MUSCLES OF THE HEAD 111 



The depressor mandihulce consists of two parts which 

 have been termed digastric and biventer. It connects the 

 under surface of the occiput with the lower jaw. 



Osteology ' 



Vertebral Column. — Highly characteristic of birds is the 

 saddle-shaped (' heterocoelous ') form of the vertebral centra. 

 The existence of this mode of articulation, though confined 

 to birds, is not found everywhere and in all of the existing 

 members of the order, and is not found at all in some of the 

 extinct forms, Archceopteryx and Ichthyornis. As to exist- 

 ing birds, the opisthocoelous form is frequently met with, but 

 so irregularly as not to be of much use from a classificatory 

 point of view. The existence of such vertebrae, pointing 

 towards reptiles, may, however, be accepted as some indica- 

 tion of an archaic position in the order. The matter has 

 been lately summed up by the late Professor Paekbr,^ from 

 his own investigations, and from those of others. 



The dorsal vertebrEE have been found to be opisthocoelous 

 in penguins and auks, in Limicolse- (including Laridae), but 

 not in petrels ; among the parrots Paekbe met with this 

 ■condition in several forms, where it was found to be com- 

 bined with terminal epiphyses, a character which is just 

 commencing in the lowest mammalia (Ornithorhynchus) and 



' The following are a few works which deal with the general osteology of a 

 number of forms. More special treatises will be referred to in their proper place :^ 

 E. Blanchakd, ' Eecherches sur les Caraotdres Ost^ologiques des Oiseaux,' 

 <fec., Ann. Sci. Nat. xi. 1860, p. 11 ; J. F. Bkandt, ' Beitrage zur Kenntniss der 

 Naturgeschichte d. Vogel,' &o., Mem. Acad. Sci. St. Petersb. (6), iii. 1840 ; 

 Eyton, Osteologia Avium, London, 1858-81 ; A. B. Meyee, Abbildvngen 

 von Vogelskeleten, Dresden, 1879-96 ; Milne Bdwakds and Geandidieb, Hist. 

 Nat, dc, de Madagascar, ' Oiseaux,' Paris, 1879-85 ; C. L. Nitzsch, Osteogra- 

 phische Beitrage, &a., Leipsic, 1811 ; P. Gbevais, ' Description Ost^ologique de 

 I'Hoazin, du Kamichi,' &c., Zool. in Voy. de Gastelnau, Paris, 1855. Besides 

 FiJEBKiNGEE, Untcrsuchungen zur Morphologie v. Syst. d. VSgel, and Gadow, 

 ' Aves,' in Bronn's Klassen v. Ordmmgen des Thierreichs, for brief and largely 

 osteological definitions of birds see Seeeohm, Classification of Birds, London, 

 1889, and Shaepe, Osteological Catalogue of College of Surgeons Museum, 

 London, 1891. 



2 ' On the Vertebral Chain of Birds,' Proc. Boy. Soc. xliii. 1888, p. 465. 



