100 COMPARATIVE ANATOMY 



this constricts the vessels, and the antler, being deprived of nutriment, falls 

 off periodically at the close of the breeding season. In the young animal, 

 the antlers are simple, but year- by year they become more complicated and 

 branched. Giraffes possess persistent antlers covered by hair and without any 

 process from the frontal : they do not become anohylosed to the latter bone. 

 The differentiation into horned and antlered forms first began in the 

 Miocene epoch. 



In the nasal cavity, scroll-like turhinals ^ are present : these are 

 preformed in cartilage, and unite with the surrounding bones. The 

 two nasal chambers are separated from one another by a cartilaginous 

 septum nasi, the posterior part of which becomes ossified by a 

 vertical plate (mesethmoid) in connection with the lamina cribrosa. 

 The vomer, which is unpaired in the adult, is situated immediately 

 below the nasal septum. Cartilage is retained only in the latter 

 region and around the external nostrils (" aliseptal " and " alinasal 

 cartilages "). The nasal cavities communicate anteriorly with the 

 mouth by means of the incisive or naso-palatine canals as well as 

 posteriorly by the internal nostrils. 



As regards the structure of the hard palate, Mammals 

 agree essentially with Crocodiles, but the small pterygoids (except, 

 e.g., in Anteaters and some Cetaceans) do not take part in its 

 formation. The palate is very long in Echidna and in certain 

 Edentata and Cetacea, and often (e.g., Marsupialia) presents 

 unossified vacuities. 



The premaxiDa takes an important part in enclosing the 

 nasal cavity : it also contributes to the hard palate, and surrounds 

 the nasopalatine canal. In the lateral parts of the face of most 

 Mammals, the jugal or malar connects the maxilla with a process 

 of the squamosal (instead of with the quadrate, as in Amphibia 

 and Sauropsida) : thus a zygomatic arch is formed from these three 

 bones (Fig. 79). In most cases {e.g., Ungulata and Primates) the 

 jugal is also connected with a process of the frontal, and thus the 

 orbit becomes more or less completely separated from the temporal 

 fossa. 



The tympanic membrane is connected with the membrane of 

 the fenestra ovalis by an articulated chain of small auditory 

 ossicles, extending across the tympanic cavity and consisting of the 

 malleus, incus (with its orbicular apophysis), and stapes — instead of 

 by a single columella as in Amphibians and Eeptiles. The two 

 former of these bones arise in the embryo from the proximal 

 end of the mandibular arch, one portion of which becomes 

 constricted off to form the incus and another the malleus, 

 both portions afterwards becoming ossified. The part of the arch 

 in the lower jaw, distal to the malleus, corresponds to Meckel's 

 cartilage, and in Fig. 80 the two are seen still in continuity. 

 The stapes, which is stirrup-shaped in all Mammals but Mono- 

 tremes and certain Marsupials and Edentates, plugs the fenestra 



' For details of the turbinals in JIammals and other Vertebrates, compare the 

 section on the olfactory organ. 



