TEMPORO-MAXILLARY ARTICULATION. 



nally, by a prominent ridge or process. This 

 is the case in the higher Quadrumana, and 

 there is an indication of it even in the lower 

 races of mankind. In the Cin-nivora the con- 

 dyle is in general extremely long, cylindroid, 

 and its length is set, almost or quite directly 

 from side to side, whilst the surface to which 

 it is opposed is bounded by a very salient 

 ridge fore and aft. This condition attains its 

 acme in the Badger, where the salient ridges, 

 especially the posterior, even arch over and 

 surround the cylindroid condyle so much, that 

 in the dry skull the lower jaw remains at- 

 tached to the cranium without any artificial 

 contrivance, and can be removed only by 

 slipping each half out sideways when the sym- 

 physis has been disjoined. With such a joint 

 there cannot, of course, be the usual lateral 

 motion of the jaw. In the Rodentia, on 

 the contrary, the long diameter of the con- 

 dyle is directed antero-posteriorly, and is ad- 

 apted to an antero-posterior groove, mostly 

 bounded, internally and externally, by salient 

 ridges. Hence that nibbling, antero-posterior 

 motion of the jaw, which is so conspicuous 

 in these animals, and with which their teeth 

 and masticatory muscles are in such admirable 

 relation. This only holds good in respect of 

 the placental rodents. In that great marsupial 

 rodent, the Wombat, the saliently arched, 

 cylindroid condyles have the usual side to 

 si le and converging-behind set of their long 

 diameters. The articular surface which is 

 opposed to this condyle is placed, as in the 

 plaeental rodents, on the zygomatic process, 

 which in both runs almost directly outwards; 

 instead, however, of being an antero-posterior 

 groove it is a transverse convex cylindroid, 

 describing a retreating curve ; so that in the 

 tcmporo-maxillary joint of the wombat a cylin- 

 droid ridge is opposed to another cylindroid 

 ridge, the one describing a salient, the other a 

 retreating arch.* The articular facets of the 

 temporo-maxillary joint in the Ruminant/a 

 approach more nearly to plane surfaces than 

 in any other animals. The squamosal facet 

 is bounded by a ridge posteriorly, but has 

 no bony limitation in front. This is in re- 

 lation with the extensive lateral movement 

 of the jaw which these animals use in chew- 

 ing the cud, and with the limited power of 

 gaping which they possess. The rest of the 

 mammalia present varieties in this joint which 

 are extremely interesting, but an account of 

 them would occupy more space than the limits 

 of this work permit. 



Aves. In birds, as above stated, the arti- 

 cular surface of the lower mandible is con- 

 cave, and is adapted to the tympanic bone, or, 

 as the older ornithologists called it, the os 

 quadratum.f The articular surface of the 

 lower jaw presents two concave depressions. 

 The tympanic bone is itself moveable, being 

 articulated to the cranium by two diarthrodial 

 joints, so that a bird's lower jaw is swung to 

 the head by two moveable jointings. 



* See Art. MAKSUPIAI.IA, iig. 

 f Soe Art, AVES. 



94. 



Rcplilia. The evenly concave articular 

 surface of a reptile's lower jaw is often con- 

 tributed to by more than one of the osseous 

 pieces which compose the inferior maxilla of 

 the oviparous vertebrates. In the crocodile 

 by two, in some chelonia by three of these 

 pieces. The tympanic bone is articulated 

 with the other cranial bones by suture, and 

 is therefore immoveable, in the Crocod/lin and 

 Chelonia, but it is articulated with them by 

 diarthrosis, and therefore moveable, in the 

 Laccrtia, Op/tidia, and lialracliia. In some 

 ophidia, as the Python, there being, properly 

 speaking, no symphysis of the lower jaw, but 

 instead of it an elastic ligament, holding the 

 two halves together at the chin, one temporo- 

 maxillary joint is capable of movement inde- 

 pendently of the other. In the Batrachia, 

 among their many fish-like characters is that 

 of a dismemberment of the tympanic bone, 

 which consists of an upper and a lower piece. 

 Pisces. In the majority of fishes, the 

 tympanic bone is represented by four separate 

 pieces called epi-, hypo-, meso-, and prae- 

 tympanic bones. These, further, bear upon 

 their posterior edge three opercular bones, 

 which were considered by GeorFroy St. Hilaire 

 to be the homologues of the ossicula auditus, 

 but are regarded by Prof. Owen as append- 

 ages, serially homologous with the costal 

 appendages. The uppermost piece, the epi- 

 tympanic, articulates by a diarthrodial joint 

 with the mastoid, and the lowermost, hypo- 

 tympanic, presents a diarthrodial convexity to 

 the lower jaw. The four tympanic pieces arti- 

 culate with one another, and with the oper- 

 cular bones by the interposition of ligament, 

 or rather membrane, connecting their thin ad- 

 jacent edges together, so that the whole appa- 

 ratus is capable of bilging outwards in the 

 movements of respiration. In the eel tribe 

 (Murtemdae) the number of tympanic bones 

 is reduced to three, which is obviously an 

 approach towards the two tympanic pieces of 

 the Batrachia. The formation of the cranial 

 part of the joint in question by the pterygoid 

 and tympanic bones conjointly, in the Lepido- 

 siren has been mentioned above. In the Sharks 

 and Rays the tympanic pedicle descends upon 

 that part of the pterygoid which forms the 

 joint, but does not actually reach the articula- 

 tion, the pterygoid being interposed. In these 

 fishes, the super-maxillary and pterygoid pieces 

 being confluent, and both bearing teeth, it ap- 

 pears at first sight that the whole upper jaw is 

 formed by the former alone, consequently that 

 the inferior maxilla is articulated directly with 

 the super-maxillary ; such an articulation, 

 however, never takes place in any animal. 



The interarticular fibro-cartilctge is con- 

 stantly met with in Mammalia, but in neither 

 of the other vertebrate classes. 



Homology of the joint. The joint in ques- 

 tion is the articulation between the plcura- 

 pojyhysis and the hcemapopliysis of the frontal 

 vertebra. It is therefore serially homologous 

 with the joint between the rib and the costal 

 cartilage of mammalia, or the vertebral and 

 sternal ribs of birds and reptiles. 



