24 STRUCTUEE AND EC0N031Y OF THE HORSE. 



from each other. The squamous portion is externally a convex 

 plate, with a hooked projection arising from it ; this process 

 assists in forming the zygomatic arch. The squamous portion 

 affords at the posterior part a shallow cavity for the^ articulation 

 of the lower jaw bone. This glenoid cavity, as it Is termed, is 

 much deeper In carnivorous animals, who require to open their 

 jaws more extensively ; and an inspection of this portion of the 

 skeleton alone will enable the comparative anatomist to decide 

 to what order the animal might have belonged. In herbivorous 

 races a grinding lateral motion of the jaws only is required, and, 

 accordingly, the articulation is wide and shallow. The zygo- 

 matic arch, too, is much more arched in the carnivora, In order 

 to afford more room for the development of the temporal muscle 

 which governs the jaws, than is required for the more moderate 

 exertions of herbivorous animals. 



Tlie petrous portion of the temporal bone, so called from its 

 rocky nature. Is apparently a solid convex figure. It contains, 

 however, the organ of hearing, and has on its internal surfiico 

 orifices for the passage of the auditory nerve, and on the external 

 part we find a larger orifice for the passage of sound. The 

 internal structure of this bone is as beautiful as it is curious ; 

 possessing vestibules and canals for the ramification of the nerve, 

 and a singular cavity, having a communication with the mouth, 

 in which are discovered four diminutive bones, with their corre- 

 sponding muscles, which serve the purpose of propagating and 

 modifying the sound. 



The inferior and middle parts of the cranium are formed 

 principally by the Sphenoid, a bone which somewdiat resembles 

 a bird in flight, having a body and four processes, two of which 

 are called the wings, and two the legs. This bone supports the 

 middle lobes of the cerebrum, and presents several holes and 

 depressions for the passage of nerves. 



The cranial cavity is separated from the nasal by the Ethmoid 

 bone, which also somewhat resembles a bird in flight, but with- 

 out legs, and is situated in front of the bone last described. It 

 supports the anterior lobes of the cerebrum, and has holes for 

 the exit of the olfactory nerves ; and on its internal and inferior 

 surface it forms cavities, called the ethmoidal sinuses, which are 

 separated by a bony septum from each other, and are perforated 

 by a vast number of small holes for the passage of the olfactory 

 nerves to tlie nasal cavities. 



Such is a very brief, and consequently imperfect, description 

 of the bones composing the brain-case, which are arranged in a 

 form at once the most compact and most durable ; so that the 

 noblest bridge which spans our rivers, or the finest dome that 

 surmounts our cathedrals, is inferior to the mechanism displayed 

 in the simple construction of the skull. 



