130 COMPARATIVE ANATOMY 



(opisthotic) portion reaches the surface of the skull between the 

 exoccipital and the tympanic bone, the homology of which is open 

 to doubt, but which possibly corresponds to the paraquadrate or 

 quadratojugal. The tympanic overlies the petrous portion of the 

 periotic, and gives attachment to the tympanic membrane: in 

 the Placentalia it forms the tubular external auditory passage or 

 meatus below which it may expand into a lulla tympani, which 

 encloses the tympanic cavity and communicates with the pharynx 

 by means of the Eustachian tube. The " temporal bone " of 

 human anatomy represents the fused periotic, tympanic, and 

 squamosal, the two last of which are investing bones. 



The cranial cavity is roofed in by frontals, parietals, and a 

 supraoccipital : a primarily paired interparietal, not preformed in 

 cartilage, may remain distinct or may unite with the supraoccipital 

 or frontals. These roofing elements, like many of the other cranial 

 bones, are united by sutures which usually persist, at any rate for 

 a long time. Many of the bones are more or less spongy internally, 

 and may contain definite air-sinuses (e.g. in Ungulates). 



The parietals and frontals vary much as regards form and 

 relative size in the different orders. In Primates, amongst many 

 others, the parietals are well developed, while in Insectivores they 

 are small : in toothed Whales they become separated from one 

 another by a large bone formed by the fusion of the supra- 

 occipital and interparietal, which reaches to the frontal (Fig. 94). 

 In many Mammals there is a large parietal and supraoccipital 

 crest in correlation with the strongly-developed muscles of the jaws 

 and neck. The frontals, which, like many of the neighbouring 

 bones, may become united together, extend downwards towards the 

 orbital region and cribriform plate, and thus take part in forming 

 the walls of the cranium and orbit. 



Most of the true Ruminants are provided with horns or antlers 

 projecting from the frontal bones, the formation of which is to be 

 traced primarily to the integument (Fig. 93). 



In the Cavicornia (Bovinae, Antelopinse, Caprinse, Ovinse) 

 bony processes arise from the frontals, which become hollow and 

 are enveloped by horn formed from the epiderm. They are usually 

 present in both sexes, but in Tragelaphus, Neotragus, and others 

 are absent in the female. In the Cermdce. a solid integumentary 

 bone is developed and becomes united with the frontal, growing 

 out to form the antler. After attaining its full development, 

 the investing skin dries up owing to the development of the 

 " burr " at its base ; 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 or less complicated and 

 branched. They are confined to the male except in the case of the 

 Reindeer. Amongst Giraffes, in addition to a short median " frontal 

 horn " present in many of the sub-species, both sexes possess small 



