66 THE ANATOMY OF THE HORSE. 



wings, neck, and appendages. The body, which is some- 

 what concealed, being hollow because of the ethmoid 

 sinuses {Plate Yll. f) , which are cavities within it; upon 

 the wings are parts of the optic, and the lateral nasal fora- 

 mina. Towards the nose are two fine flat pieces of bone, 

 pierced by numerous holes ; these are the cribriform plates, 

 that give passage to the Olfactory nerves. In front of these 

 grow, within the nasal cavities, a spongy formation, termed 

 the ethmoid cells {Plate VII. e) . The internal surface is 

 smooth, and between the two cribriform plates is a slight 

 projection of bone, termed the crista galli, which gives 

 attachment to the falx cerebri. 



Os TRiQUATRUM (single) is an angular-shaped bone ; 

 smooth upon its outer surface. It joins the occipital bone, 

 and lies before it ; being in the middle or upon the top of 

 the skull ; its inner surface is indented for various sinuses, 

 and from it proceeds a projecting piece of bone, called the 

 tentorium {Plate VII. h), which divides the cerebrum from 

 the cerebellum. 



Parietal bones (a pair) are situated upon the sides of the 

 skull. Externally they are smooth and convex, being 

 covered by the temporal muscles to which they give origin ; 

 internally they are concave, and indented by the convo- 

 lutions and the arteries of the brain. 



Squamous temporal (a pair) are in front of the petrous 

 temporal. External surface. Projecting pieces of bone upon 

 the outer sides are called the zygomatic processes. The 

 orbital arches rest upon part of these processes ; upon the 

 inferior borders are the glenoid cavities for articulating with 

 the lower jaw, near to which are the mastoid processes. 

 The internal surface is smaller than the outer, being trenched 

 upon by the parietal bones. 



Frontal bones (a pair). External surface. These bones 

 form the arches of the orbit ; the arch is pierced by the 

 supra orbital foramen, and the under part of the arch is 

 hollow for the lacrymal gland ; the frontal also forms half of 

 the lateral nasal foramen ; it enters into the composition of 

 the top of the nose. The internal surface is divided into 

 two parts ; one belongs to the nose, the other to the cra- 

 nium. The external and internal plates of the bone sepa- 

 rate, and thus form the frontal sinuses {Plate VI. and 



