68 THE SKELETON OF THE HORSE 



Development. — Each nasal bone ossifies in membrane from a single center. ^ 

 The nasal suture does not close completely even in old age. In some cases the two 

 plates separate to inclose a small air-space (nasal sinus) in the posterior part. 



The Lacrimal Bones 



The lacrimal bones (Ossa lacrimaha) are situated at the anterior part of the 

 orbit, and extend forward on the face to the posterior border of the maxilla. Each 

 presents three surfaces and a circumference. 



The lateral face is clearly divided into orbital and facial parts by the orbital 

 margin. The orbital surface (Facies orbitalis) is triangular in outline, smooth and 

 concave; it forms part of the medial and front wall of the orbit. Near the orbital 

 margin it presents a funnel-like fossa (Fossa sacci lacrimalis), which is the entrance 

 to the lacrimal canal; the fossa is occupied by the lacrimal sac, which is the dilated 

 origin of the naso-lacrimal duct. Behind this is a depression in which the inferior 

 oblique muscle of the eye takes origin. The facial surface (Facies facialis) is more 

 extensive, and has the form of an irregular pentagon. It is slightly convex and 

 smooth in the foal, flattened in the adult. It usually bears the small lacrimal 

 tubercle, which is situated nearly an inch (ca. 2 cm.) from the orbital margin. 

 The orbital margin (Margo orbitalis) is concave, rough above, smooth below. 



The nasal surface (Facies nasalis) faces into the frontal and maxillary sinuses. 

 It is concave and very irregular, and is crossed almost horizontally by the osseous 

 lacrimal canal (Canalis lacrimahs osseus). 



The circumference articulates dorsally with the frontal and nasal bones, 

 ventrally with the malar and maxilla, in front with the maxilla, and behind with 

 the frontal. The various sutures so formed are designated by combinations of the 

 names of the bones. 



Development. — Each ossifies in membrane from a single center. 



The Malar Bones 



The malar or zygomatic bones (Ossa zygomatica) are placed between the lac- 

 rimal above and the maxilla below and in front. Each is irregularly triangular in 

 outline and presents three surfaces, three borders, and a process. 



The facial surface (Facies facialis) is smooth, slightly convex, wide in front, 

 and narrow behind. At its lower part it presents the facial crest, which is continu- 

 ous in front with the similar ridge on the maxilla and behind with the zygomatic 

 process of the temporal; the crest is rough below, where the masseter muscle is 

 attached to it. 



The orbital surface is much smaller than the facial surface, from which it is 

 separated by the concave orbital margin (Margo orbitalis). It is concave and 

 smooth, and forms part of the lower and front wall of the orbit. 



The nasal surface is concave and faces into the maxillary sinus. In the young 

 foal a considerable part of it articulates with the maxilla. 



The dorsal border articulates with the lacrimal chiefly, but to a small extent 

 behind with the maxilla also. 



The ventral and anterior borders articulate with the maxilla. 



The posterior extremity is formed by the zygomatic process, which is beveled 

 above and is overlapped by the zygomatic process of the temporal bone. 



Development. — Each ossifies in membrane from one or two centers. 



The Turbinate Bones 



These (Ossa turbinata) are delicate, scroll-like bones, four in number, which 

 are attached to the lateral walls of the nasal cavity. They project into the cavity 

 and greatly diminish its extent. Each is composed of a very thin lamina, cribri- 



