THE STERNUM—THE SKULL 157 
inch long. The fourteenth rib, when present, is also floating. As compared with 
those of the ox, they are narrower and are more strongly curved in the anterior part 
of the series. The lateral surface is in general smooth and rounded. The second 
to the eleventh form diarthroses with their cartilages. 
The thirteenth rib may be more or less rudimentary on one side or both, and may be fused 
with the corresponding vertebra; the latter may, therefore, be ambiguous in character. 
THE STERNUM 
The sternum resembles in general that of the ox. The number of segments 
may be reduced to six, and the primitive division of the next to the last sternebra 
into two lateral halves may persist for along time. The first segment is cylindrical, 
with enlarged ends; the second and third are wide and flat; the last is long and nar- 
row. 
THE SKULL 
The more important differences in the skull of the sheep as compared with 
that of the ox are in regard to the cranium. Viewed from above, the cranium is 
irregularly hexagonal in outline; it is widest in the frontal region, between the 
posterior parts of the orbits, and narrows greatly both anteriorly and posteriorly. 
In profile the roof of the cranium is strongly convex; the highest part of the curve 
coincides with the greatest width, and the posterior part slopes at an angle of about 
45 degrees with the basal plane. 
The occipital bone forms all of the nuchal surface of the cranium, except a 
small lateral area occupied by the mastoid part of the temporal bone. A narrow 
part (about 1.5 cm. in width) enters into the formation of the roof of the cranium 
also; it joins the parietal bones at a transverse suture. The parietal and nuchal 
surfaces are separated by a rough transverse ridge, the central part of which is 
united below with the external occipital protuberance, to which the ligamentum 
nuche is attached. The mastoid foramen is situated between the lateral border 
and the petro-mastoid part of the temporal bone. The paramastoid process is 
grooved laterally, has a concave anterior border, and tapers to a blunt point. The 
basilar part is wide; the tubercles at its junction with the sphenoid are placed 
laterally, and are broad and short. 
The sphenoid bone resembles that of the ox. The posterior wall of the deep 
hypophyseal or pituitary fossa is formed by a plate (Dorsum sellz) which is di- 
rected forward and upward, and bears a projection (Processus clinoideus posterior) 
at each side of its upper part. The sphenoidal sinus is commonly absent or rudi- 
mentary. 
The ethmoid bone resembles that of the ox. 
The parietal bone fuses soon after birth with its fellow and with the inter- 
parietal. From this union there result a central quadrilateral curved plate which 
forms part of the roof of the cranium, and, separated from it by a curved line, a 
narrower lateral part, which extends forward on either side as part of the medial 
wall of the temporal fossa. The frontal sinus does not extend into the parietal bone. 
There is no internal occipital protuberance. 
The frontal bone is relatively less extensive than in the ox. The naso-frontal 
part is strongly curved, but varies considerably in contour in different breeds. In 
horned breeds the processus cornus projects from the lateral part of the external 
surface a little (ca. 1.5 em.) behind a transverse plane through the posterior margin 
of the orbit. The process varies in size and shape, and does not contain an extension 
of the frontal sinus. In other cases there is a rounded tuberosity or a slight rough- 
ened elevation. The supraorbital foramen is further forward than in the ox, being 
just behind a transverse plane through the middle of the orbit; it is a little nearer 
