78 THE SKELETON OF THE HORSE 
above it. The foramen opens forward. and through it the infraorbital artery and 
nerve emerge. The surface over the premolar teeth varies greatly with age, in 
conformity with the size of the embedded parts of the teeth. In the young horse 
the surface here is strongly convex, the outer plate of bone is thin and even defective 
sometimes in places, and the form of the teeth is indicated by eminences (Juga 
alveolaria). In the old animal the surface is concave on account of the extrusion 
of the teeth from the bone. The downward curve of the premaxilla is pronounced 
in the young subject, very slight in the aged. 
In some skulls there is a distinct depression a short distance in front of the orbit; here the 
levator labii superioris proprius arises. 
The ventral or basal surface (Norma basalis), exclusive of the mandible, 
consists of cranial, choanal, and palatine regions. 
The cranial region (Basis cranii externa) extends forward to the vomer and 
pterygoid processes (Fig. 50). At its posterior end is the foramen magnum, flanked 
by the occipital condyles. Lateral to these is the condyloid fossa, in which is the 
hypoglossal foramen, which transmits the hypoglossal nerve and the condyloid 
vein. Further outward are the paramastoid processes (Processus jugulares) 
of the occipital bone. Extending forward centrally is a prismatic bar, formed 
by the basilar part of the occipital and the body of the sphenoid bone; at the 
junction of these parts are tubercles for the attachment of the ventral straight 
muscles of the head. On either side of the basilar part of the occipital bone is the 
foramen lacerum (basis cranii), bounded laterally by the base of the petrous tem- 
poral bone. In front of these the region becomes very wide on account of the lateral 
extension of the zygomatic processes, which present ventrally the condyle and 
glenoid cavity for articulation with the mandible. Beyond this the process turns 
forward and joins the zygomatic process of the malar, completing the zygomatic 
arch and the surface for the attachment of the masseter muscle. On either side of 
the body of the sphenoid is the infratemporal fossa, formed by the temporal wing 
and the root of the pterygoid process of the sphenoid bone. It is bounded in front 
by the pterygoid crest, which separates it from the orbit and the pterygo-palatine 
fossa. In it is the alar canal, which transmits the internal maxillary artery. A 
little lower is the entrance to the pterygoid canal. 
The choanal region presents the pharyngeal orifice of the nasal cavity. This 
is elliptical and is divided in its depth medially by the vomer into two choane or 
posterior nares. It is bounded in front and laterally by the palate and pterygoid 
bones, behind by the vomer. It is flanked by the hamular process of the pterygoid 
bone. The plane of the opening is nearly horizontal, and the length is about 
twice the width. 
The palatine region comprises a little more than half of the entire length 
of the base of the skull (Fig. 48). The hard palate (Palatum durum) is concave 
from side to side, and in its length also in the anterior part. It is formed by the 
palatine processes of the premaxillze and maxillee, and the horizontal parts of the 
palatine bones. It is circumscribed in front and laterally by the alveolar parts of 
the maxilla and premaxilla. The interalveolar space (Margo interalveolaris) is that 
part of the arch in which alveoli are not present. Behind the last alveolus is the 
alveolar tuberosity, and medial to this is a groove for the palatine vein. In the 
middle line is the median palatine suture (Sutura palatina mediana). In the line 
of the suture, a little behind the central incisors, is the foramen incisivum, through 
which the palato-labial artery passes. On either side, parallel with the alveolar 
part of the maxilla, is the palatine groove (Sulcus palatinus), which contains the 
palatine vessels and nerve. It is continuous at the anterior palatine foramen with 
the palatine canal, which is situated between the maxilla and the palatine bone. 
The palatine fissure is the narrow interval along the lateral margin of the palatine 
