ARTICULATIONS IA T BIRDS. 173 



do not unite by their bodied in the form of a continuous scries of amphiarthroses, 

 as in tin' domesticated iiiummals; but that instead <>i tin-si- mixed articulations there are 

 -If diarthivsi >, which may be included in the class created by Criiveilhicr under 

 tin- title nt' nrt'i'ul/iliiiit Inj n-rijiroail Itall and socket, each veitfbra Incoming co'in- -t- '1 

 with tlie adjaei -iit vertebra- by means of facets convex in one sense and concave in tho 

 M -ii -i- periH'iidioilar to the first. These facets ore manifestly covered by cirtila 

 incrui-tution ; and it appears that, in-trad of their being applied directly agninst the 

 oppositi t'aret-, which present a precisely inverse conformation, they are separated by 

 an extremely thin fibro-cartilaginous disc, which resemble* the interosseous meniscus of 

 inporo-maxillary articulation in the Carnivora of the Cnt species. Two loose 

 f-ynoviul capsules, separated by this interarlicnlar lamina, complete the framework of 

 tach urticultition, and favour the play of the vertebra on one another. This arrange- 

 ment has only, so far as we are aware, been observed in the swan, and that very 

 imperfectly ; but it probably belong.s to the entire class of birds, for until now we have 

 iin-t with it in all the individuals submitted to examination. 



In its dorso-lumbar and sacral portion, the spine is a single piece, in consequence of 

 the consolidation of the vertebra, and does not show any proper articulations. 



In the coccygeal region, the mobility of the spine re-appears ; but it is far from being 

 BO marked as in the cervical region ; the vertebra here are united by amphiarthrosis, and 

 not by nriprocal ball and socket. 



Occipito-atloid articulation. It has been shown that there is only one more or less 

 spheroidal condyle of the occipital bone, and a single cavity on the anterior margin of 

 the spinal canal of the atlas. The occipito-atloid articulation is therefore a true enar- 

 throsLs, with varied and very extensive movements; a disposition which accounts for 

 the facility with which birds can pivot their heads on the superior extremity of the 

 vertebral stalk. 



Temporo- maxillary articulation. The play of this articulation offers one peculiarity 

 in that it causes, during the separation of the mandibles, not only the depression of the 

 inferior, but also the elevation of the superior mandible. The arrangement which permits 

 this movement has been already made known ; but yet it is difficult to understand, be- 

 cause there is no active agent, no proper muscle to directly effect it. Nevertheless, the 

 mechanism which executes it is most simple, and may be given in a few words : Thus, 

 wo know that the square bone, interposed between the temporal and maxillary bones, 

 like the iuterarticular meniscus of mammals, is united outwardly with the jugal bone, 

 find inwardly with the pterygoid. We know also that the latter rests, by means of a 

 diarthrodial facet, on the body of the sphenoid, and that it abuts against the posterior 

 extremity of the palate bones; while the first, the zygomaticus, is joined directly 

 to the supermaxillary bone. The superior jaw, it is also known, is movable on 

 the cranium, because of the flexibility of the cartilages or bony plates uniting these two 

 portions of tlie head. It may then be added, that the square bone receives on its anterior 

 n one or two small muscles which are attached to the base of the cranium, and that 

 these bones may be pushed, or rather drawn forward, by the contraction of these muscular 

 fasciculi. It is this projecting, or pushing, transmitted to the up|x-r mandible through 

 the medium of the jugal bone on the one side, and the pterygoid bone on the other, that 

 produces the elevation of that mandilile. 



iiin.L' is easier than to prove it ; it is only necessary to take the head of a bird, 

 denude it of all its i-olt parts, and press with the fingers behind the two square bones, 

 t imitate the action of tne elevator muscles; we then see the internal extremity of the 

 pt. rv_'oid Iwne glide on the facet of the sphenoid, and push before it the palatine bone. 

 during which the zygomatic bone acts in the same manner on the maxillary ; and in this 

 wa> is produced, through the influence of this postero-auterior propulsion, the ascending 

 movement we undertook to explain. 



THIRD SECTION. 



THE MUSCLES. 



AFTER the study of tho bony levers and their articulations, comes tho 

 liption of tho agents whoso function it is to move them. These arc tho 

 /, fibrous organs possessing the property of contracting uinU-r tin 

 inline nei- of a stimulus. 



