ARTICULATIONS IN BIRDS. I73 



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 

 veritable diarthroses, which may be included in the class created by Cruveilhier under 

 the title of articulation hy reciprocal ball and socket, each vertebra becoming counected 

 with the adjacent vertebrae by means of facets convex in one sense and concave in the 

 sense perpendicular to the first. These facets ore manifestly covered by cjrtilage of 

 incrustation ; and it appears that, instead of their being applied directly 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 tlris interarticular lamina, complete the framework of 

 each articulation, and favour the play of the vertebrae on one another. This arrange- 

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

 imperfectly; but it probably belongs tu 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 consequence of 

 the consolidation of the vertebr'se, 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 vertebrse here are united by amphiarthrosis, and 

 not by reciprocal ball and socket. 



Oocipito-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 

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

 throsis, 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 tlie 

 vertebral stalk, 



Temporo-maxiUary articulation. — The play of this articulation offers one peculiarity 

 in that it causes, during the separation of the mandibles, not oirly 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 diihcult 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: Tims, 

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

 like the interarticular meniscus of marimnals, is united outwardly with the jugal 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 palate bones ; while the first, the zygomaticus, is joined dir'ectly 

 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 tire head. It may then be added, that the square bone receives on its arrterior 

 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 tlie contraction of these muscular 

 fasciculi. It is this projecting, or pushing, transmitted to the upper mandible through 

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

 ijroduces 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 the two square bones, 

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

 pterygoid bone 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 

 way is produced, through the influence ot this postero-anterior propulsion, the ascending 

 movement we undertook to explain. 



THIED SECTION. 

 The Muscles. 



After the study of the bony levers and their articulations, comes the 

 description of the agents whose function it is to move them. These are the 

 muscles, fibrous organs possessing the property of contracting under the 

 influence of a stimulus. 



