226 THE ARTICULATIONS. 



be remarked that these do not unite by their bodies in the form of a continuous 

 series of amphiarthroses, as in the domesticated Mammals ; but that, instead of 

 these mixed articulations, there are real diarthroses, which may be included in the 

 class created by Cruveilhier under the title of articulation hij reciprocal ball-and- 

 socket — each vertebra becoming connected with the adjacent vertebra by means of 

 facets, convex in one sense and concave in the sense perpendicular to the firat. 

 These facets are manifestly covered by cartilage of incrustation ; and it appears 

 that, instead of their being applied dii'ectly against the opposite facets, which 

 present a precisely inverse conformation, they are separated by an extremely thin 

 fibro-cartilaginous disc, which resembles the interosseous meniscus of the temporo- 

 maxillary articulation in the Carnivora of the Cat species. Two loose synovial 

 capsules, separated by this interarticular lamina, complete the framework of each 

 articulation, and favour the play of the vertebrae 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 belongs to the entire class of Birds, for until now 

 we have met with it in all the individuals submitted to examination. 



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

 sequence of the consolidation of the vertebrae, and does not show any proper 

 articulations. 



In the coccygeal region, the mobility of the spine reappears, but it is far 

 from being so marked as in the cervical region ; the vertebrae here are united by 

 amphiarthrosis, and not by reciprocal ball-and-socket. 



Occiinto-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 there- 

 fore a true enarthrosis, 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 vertelbral stalk. 



Temporo-maxillary articulation. — The play of this articulation offers one pecu- 

 liarity, 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 alluded to (p. 189) ; 

 but yet it is difficult to understand, because 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, we know that the square 

 bone, interposed between the temporal and maxillary bones, like the interarticular 

 meniscus of Mammals, is united outwardly with the malar bone, and 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 palatine bones (Fig. 109) ; while the first, the zygomaticus, is joined 

 directly to the supermaxillary bone. The upper 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 the head. It may then be added, that the square bone 

 receives on its anterior process 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 muscles. It is this projecting or pushing, 

 transmitted to the upper mandible through the medium of the malar bone on the 

 one side, and the pterygoid bone on the other, that produces the elevation of that 

 mandible. Nothing is easier than to prove it ; it is only necessary to take the 

 head of a Bird, denude it of all its soft parts, and press with the fingers behind 



