6 ELEMENTARY SKETCH OF THE OSTEOLOGY OF BIRDS. 



is blunter, passes upwards and backwards, and fits loosely into 

 a hollow in the squamosal (sq. fig. 2)'^ element of the skull. 



The squamosal bone is that which surrounds the aperture of 

 the ear, and with which are closely connected the petrosal] and 

 tympanic bones which contain the internal ear. The squamosal 

 in birds very often gives oflF a short process which descends down- 

 wards over the quadrate. Inferiorly the quadrate bone presents 

 two oblique condyles on its anterior border, which serve for the 

 articulation of the lower jaw or mandible (m.n. pi. 1). The 

 intervention of the quadrate bone between the mandible and 

 the squamosal element of the skull is a peculiarity of the skulls 

 of birds and reptiles. In mammals the mandible always articu- 

 lates directly with the squamosal itself, and the quadrate bone 

 is supposed to be represented by the malleus\. of the internal 

 ear, which is wanting in the ear of birds, where the incus and 

 stapes^ are alone present. 



The lower jaw or mandible of birds, in its young state con- 

 sists of several distinct elements, but these with the exception 

 of the anterior or dentary piece, are firmly soldered together in 

 the adult ', the vacuity which occurs near the posterior extremity 

 of the mandible indicates, however, one of the original lines of 

 separation. The two rami (| of the mandible are almost always 

 anchylosed together anteriorly. The posterior or articular extre- 

 mity of each ramus may be either abruptly truncated, or produced 

 backwards into a curved process. In the compound nature of the 

 mandible, birds agree with reptiles and differ from mammals, 

 in which each ramus consists of a single element. 



The quadrate also articulates by an ascending process wjth 

 the prootic^ and alisphenoid^^ which are small bones 

 generally anchylosed with the squamosal, and which need not 

 occupy us further here. The antero-internal angle of the 



* Squamosal, from squama, a scale, applied, from its flat shape and articulation, 

 to the bone of the skull which articulates with the lower jaw in mammals, and sub- 

 sequently to the corresponding bone which articulates with the quadrate in birds 

 and reptiles. In human anatomy the squamosal was called the squamous part of the 

 temporal bone. 



f Petrosal and tympanic, two bones connected with the organ of hearing : the 



former named from its solid structure ( TTETpov a rock) ; the latter from its containing 

 the drum of the ear (tympanum). 



J Malleus, a hammer : apphed to the larger of the internal bones of the ear, so 

 named from its shape. 



§ Incus, an anvil ; stapes, a stirrup — names applied to the smaller bones of the 

 internal ear, and so named from their shape. The internal ear of the bird also con- 

 tains a third rod-like bone called the columella, which does not occur in mammals. 



II Ramus, a branch ; — applied to the two horizontal bars of the mandible. 



% Prootic, one of the bones forming the chain around the ear, which can only be 

 shown in foetal skulls. 



** Ali-sphenoid. See sphenoidal rostrum above. The boundaries of the all-sphenoid 

 are obliterated in the bird skull, but its general position is indicated from the association 

 of the quadrate indicated above. 



