HEAD AND NECK OF THE HORSE 109 



and stoutest laterally, where it is reinforced by an elastic band attached 

 to the pterygoid crest, it is thin where it lies against the medial wall of 

 the orbit. In connection with the medial portion of the membrane, and 

 just below the root of the zygomatic process of the frontal bone, there is 

 a flattened rod of cartilage concerned in the formation of a pully 

 through Avhich the superior oblique muscle of the eyeball plays. 



Dissection. — The periorhita must now be slit open, and, if considered 

 necessary, a portion of it may be removed. In doing this, care must be 

 exercised not to injure the structures lying close to its deep surface. 



In cleaning tlie contents of the orbit, the accumulation of fat (corpus 

 adiposum infraorl)itale) that tills all the interstices between the muscles and 

 around the eyeball, must be removed with constant regard to the fact that 

 nerves and vessels of small size are embedded in it. 



The lachrymal apparatus (apparatus lacrimalis) consists of the 

 lachrymal gland with its excretory ducts, the two lachrymal ducts, 

 the lachrymal sac, and the naso-lachrymal duct. Of these, the gland 

 is now visible. 



The lachrymal gland (glandula lacrimalis) lies under cover of the 

 zygomatic process of the frontal bone and over the dorso-lateral face of 

 the eyeball. Oval in outline and flattened in form, it is curved in 

 conformity with the shape of the orbit and the surface of the eyeball. 

 It is pale red in colour and distinctly lobulated. Twelve to fifteen or 

 sixteen small ducts (ductuli excretorii) open into the lateral half of the 

 upper fornix of the conjunctiva. 



The terminations of the frontal, lachrymal and zygomatic nerves 

 have been examined in connection with the dissection of the face 

 (page 36). The middle part of the course of these nerves is now 

 exposed. The lachrymal and zygomatic nerves follow an almost 

 parallel course within the periorbita and lateral to the muscles of the 

 eyeball. 



N. ZYGOMATICUS.— The zygomatic nerve is a branch of the maxil- 

 lary, from which it arises close to the orbital fissure. Piercing the 

 periorbita, it runs along the surface of the lateral straight muscle of 

 the eyeball, divides into two or three branches and reaches the lower 

 eyelid, in which it ramifies. Small filaments from the nerve are con- 

 nected with similar twigs from the lachrymal. 



N. OPHTHALMICUS. — The ophthalmic ^ nerve is one of the three 

 divisions of the fifth cerebral or trigeminal nerve. Leaving the 

 cranium by the orbital fissure it immediately divides into the lach- 

 rymal, frontal and naso-ciliary nerves. Two of these — the lachrymal 



1 6<pda\fx6s (ophthalmos) [Gr.], the eye. 



