Mammalia: Skin, Muscles, Skeleton 605 



the whole environment in view at the same time. The rabbit is such an 

 animal. But to the carnivore, intent upon catching a fleeing object of 

 prey, it is an advantage to look forward with both eyes because such 

 binocular vision gives a better sense of distance and relative positions 

 of objects. Accordingly, the more primitive, smaller, and weaker 

 animals usually have laterally directed eyes, while the forward-looking 

 mammals are of the stronger and more aggressive types. 



The heads of ungulates (Fig. 572) give evidence of some improve- 

 ment in cranial capacity, but the nasal region is still long. The eyes 

 are placed laterally but the region of the orbit projects more or less so 

 that vision may be more nearly forward. In most carnivores the 

 improvement is marked (Fig. 468A). The nasal region (with some excep- 

 tions) is short. The head is strongly convex on top. The distance 

 between ear and eye may be greater than that from eye to tip of snout. 

 The eyes are on the front surface of the head so that vision may be 

 directed forward. The cat has a good face and a real forehead, even if 

 somewhat of the "retreating" type. These features characteristic of 

 the larger-brained mammals approach their extreme development in 

 monkeys and apes and reach it in man. 



Sinuses. The presence of cavities in bones is common. It is, how- 

 ever, a peculiarity of the mammalian skull that certain of its bones, 

 especially in the facial region, contain particularly large air-filled 

 cavities. Each maxillary bone usually contains a large sinus (the 

 antrum) and there are commonly frontal (in man, just above the 

 eyes) and sphenoidal sinuses (Fig. 507). The existence of these 

 cavities may have some relation to the mechanical principle that a 

 hollow structure of appropriate form possesses greater rigidity than a 

 structure containing the same amount of material and of similar form, 

 but solid. These sinuses open into the nasal cavities. Any advantage 

 which they have for man would seem to be outweighed by their 

 propensity for becoming infected and making trouble. 



Visceral Arches. Such parts of the embryonic visceral skeleton 

 as lie posterior to the first or mandibular arch are represented in the 

 adult mammal in various ways. The midventral part of the hyoid 

 arch ossifies to form a hyoid bone which lies transversely at the base 

 of the tongue and immediately in front of the larynx (Fig. 468). It 

 serves for attachment of muscles which extend down the neck to the 

 thoracic region (sternohyoid and others) and as origin for an impor- 

 tant muscle, the hyoglossus, which passes forward into the tongue, 

 acting to pull the tongue downward and backward (Fig. 459). The 

 hyoid bone supports the larynx, being connected with it by a mem- 

 branous ligament. Also, some small muscles belonging to the laryngeal 

 mechanism are attached to it. The lateral parts of the hyoid arch are 



