50 THE SKELETON OF THE HORSE 



situated above the lateral portions, from which it remains distinct till the second 

 year. The outer surface is crossed by a very prominent ridge, the occipital crest ; 

 the middle part of this is thick, transverse in direction, and forms the highest point 

 of the skull when the head is in the ordinary position; laterally it becomes thinner 

 and runs downward and forward to join the temporal crest/ The crest divides 

 the surface into two very unequal parts; the small anterior area (Planum parietale) 

 presents a median ridge which is the posterior part of the external sagittal crest; 

 the large area below the crest (Planum nuchale) also has a central eminence, the 

 external occipital protuberance, on the sides of which the funicular part of the 

 ligamentum nuchse is attached. The internal surface is concave and presents a 

 deep central depression and two shallower lateral ones which adapt it to the surface 

 of the cerebellum. 



The occipital bone is connected by suture with the interparietal, two parietals, 

 and two temporals, and by synchondrosis with the sphenoid; the condyles articu- 

 late with the atlas. 



Development. — The occipital ossifies in cartilage from four centers, and con- 

 sists at birth of four pieces as described above." The lateral parts unite with the 

 basilar at three to four months, and with the squama in the second year, when the 

 bone is consolidated. 



The parieto-occipital suture and the spheno-occipital synchondrosis are ob- 

 literated about the fifth year usually. The temporo-occipital suture partially 

 ossifies in old subjects. 



The Sphenoid Bone 



The sphenoid bone (Os sphenoidale) is situated in the base of the cranium, its 

 central part lying in front of the basilar part of the occipital. It consists of a cen- 

 tral part, the body, two pairs of wings, and two pterygoid processes. 



The body (Corpus) is a cylindrical bar, flattened dorso-ventrally, and wider 

 in front than behind. Its ventral surface (Facies externa) is convex in the trans- 

 verse direction; and its anterior part is concealed to a large extent by the vomer 

 and pterygoid bones. The cerebral surface (Facies cerebralis) presents the fol- 

 lowing features: (1) In front is a raised, flattened part which is subdivided by a 

 median elevation into two shghtly concave lateral areas; this part has a posterior, 

 thin, free margin (Limbus sphenoidalis), which overlies the entrance to the optic 

 foramina. (2) Just behind this and at a lower level is a smooth transverse de- 

 pression, the optic groove (Sulcus chiasmatis), on which the optic chiasma rests. 

 (3) From each end of this groove the optic foramen passes forward and outward to 

 terminate in the posterior part of the orbital fossa. ^ (4) Near the posterior end 

 is a central depression, the hypophyseal or pituitary fossa (Fossa hypophysea), 

 which lodges the hypophysis cerebri or pituitary body. On each side of this is an 

 ill-defined groove for the internal carotid artery and the cavernous sinus. The an- 

 terior end is expanded, and is excavated to form the sphenoidal sinuses. These 

 cavities extend back as far as the optic groove, and are usually continuous in front 

 with the cavities in the vertical parts of the palate bones : they are separated by a 

 complete septum which is not always median. ■* The posterior end is flat and is 



1 The occipital crest of this description is equivalent to the external occipital protuberance 

 and superior nuchal line of man. A curved line a little lower down, which is continued on the 

 paramastoid process, represents the inferior nuchal line of man. 



2 Other terms for these parts are basioccipital (basilar part), exoccipitals (lateral parts), 

 and supraofcipital (squamous part). It sliould be noted, however, that the lines between the 

 basioccipital and exoccipitals pass through the lower part of the condjdes. 



' This foramen might well be called a canal, since it is an inch or more in length. 



* The cavity so formed may be termed the sphenopalatine sinus. The sphenoidal sinus may 

 be a separate cavity which communicates only with the ventral ethmoidal meatuses; this arrange- 

 ment exists in about a third of the cases, according to Paulli. 



