SKULL. 



487 



olfactory nerves are spread. On their outer sides these laminae 

 are connected by a thin plate of bone, the os planum, which usu- 

 ally lies as above stated against the inner wall of the maxillae. 

 Behind they are all connected together to form the sieve-like 

 cribriform plate which is joined to the hind end of the median 

 ethmoid and blocks up the anterior end of the cranial cavity. 

 The os planum and the ossified laminae constitute together the 

 lateral mass of the ethmoid bone above referred to. The laminae 

 are usually divided into two sets, which, though all parts of the 

 same bone, have received different names according to their 

 mode of attachment to neighbouring bones. These are the 

 ethmoturbinal (superior and middle turbinals, Fig. 260, C) and 



Fr 



No. 



Pe 



FIG. 260. Median longitudinal section of a sheep's skull (from Cla us). -Als alisphenoid ; O 

 ethmoturbinal, Ci maxilloturbinal, Eth ethmoid, Fr frontal, Jmx premaxilla, MX 

 maxilla, Na nasal, Ob basioccipital, 01 exoccipital, Ors orbitosphenoid, Os supraoccipital, 

 Pa parietal, Pal palatine, Pe petrosal, Ps presphenoid.FY pterysjoid, Sf frontal sinus, 

 Spb basisphenoid, Vo vomer. The median plate of the ethmoid is not seen. 



the maxillo-turbinal (Ci). The uppermost lamellae of the 

 ethmoturbinals lie immediately beneath the nasals and are fre- 

 quently united to them ; they constitute the so-called naso- 

 turbinal. The maxillo- turbinals (inferior turbinals) are the 

 portions lying farther forward : they owe their name to the 

 fact that they unite with the maxillae. The maxillo-turbinal 

 being placed farther forwards lies in the direct current of 

 respiratory air and its mucous membrane is not innervated by 

 the olfactory nerve, but by the fifth nerve. The lateral masses 

 of the ethmoid are generally held to correspond with the pre- 



