m COMPAEATIVE ANATOMY. 



are developed from the basi-sphenoid. They form the side-walls of 

 the posterior nares, and may also limit these orifices below by 

 uniting together in the roof of the palate (in Echidna, Dasypus, and 

 some Oetacea). In most Mammals they ai*e permanently distinct, 

 as they are also, for a very long time, in the Primates, before they 

 unite with the above-mentioned processes of the sphenoid to form 

 the medial lamella3 of the descending pterygoid processes. The 

 palatines generally form the inferior boundary of the posterior 

 nares, and the hinder portion of the hard palate. The ruaxillre 

 vary in length according to the extent of the facial region, and 

 always form the largest portions of the upper jaws. The premaxilla3 

 vary more considerably ; as a rule, they also take part in the forma- 

 tion of the side walls of the nasal cavity. They are rudimentary, 

 or, as compared with the maxillas, feebly developed in many 

 Chiroptera and Edentata. They bound the foramen incisivum. In 

 the Apes they fuse with the maxillas ; this union takes place so early 

 in Man that their existence was justifiably doubted for a long time. 

 The outer series of bones, which is present in the Sauropsida, and 

 which extends from the quadrate to the maxilla, is reduced in Mammalia 

 to the jugal; this bone unites the jugal process of the squamosal with 

 the maxilla, and so forms the jugal arch. In a few forms the jugal 

 is absent (Sorex), or, though united with the maxilla, does not reach 

 the jugal process (Myrmecophaga, Bradypus). When it is united 

 with a process of the frontal it gives rise to a posterior wall for the 

 orbit, and so separates this region from the temporal fossa ; there 

 are various stages of this arrangement. This process is most com- 

 plete in the Primates, where the orbital fissure represents the re- 

 mains of the wide communication which exists between the orbit 

 and the temporal fossa in other Mammals. 



In the Mammalia a tympanic bone is developed on the outer 

 face of the petrosal ; this serves as a support for the tympanic 

 membrane. It is not certain that this bone is homologous with the 

 one of the same name which we found in the Amphibia. At first it 

 always forms a bony incompletely-closed ring (annulus tympanicus) 

 (Fig. 254, at), which grows out into very various forms. In the 

 Monotremata and Marsupialia, as well as many Insectivora, etc., it 

 is never more than a simple ring. In many forms it is never 

 united to the petrosal; in the Oetacea it is very loosely so. In 

 many it forms a bony capsule which is continued into the external 

 auditory meatus. A bulla of this kind is most common in the 

 Marsupialia, Rodentia, Ferai, and in the Artiodactyla. In some 

 Marsupials, where the tympanic does not pass beyond the annular 

 condition, there is an apparently similar bulla, but this is formed 

 by an extension of the bases of the alas temporales (Dasyurus, 

 Petaurista, Perameles). When the tympanic is fused with the 

 petrosal and squamosal it takes part in the formation of the temporal 

 bone (Primates). 



