HEAD AND NECK OF THE HORSE 111 



and the frontal — may be examined at once : the naso-ciliary should 

 be dissected after the examination of the levator of the upper eyelid 

 and the superior straight muscle of the eyeball. 



N. lacrimalis. — The lachrymal nerve passes to the lachrymal gland 

 by traversing the surface of the superior rectus muscle. Its terminal 

 filaments supply the gland and the upper eyelid. A small branch 

 (ramus zygomaticotemporalis) anastomoses with a similar branch from 

 the zygomatic nerve, pierces the periorbita and appears on the fore- 

 head immediately behind the zygomatic process of the frontal bone, 

 where it joins the plexus formed by the auriculo-palpebral and frontal 

 nerves. 



N. frontalis. — The frontal nerve follows the margin of the superior 

 oblique muscle for a distance, then pierces the periorbita, and finally 

 gains the supra-orbital foramen by which it reaches the forehead, where 

 it assists in the formation of a plexus in which the auriculo-palpebral 

 and lachrymal nerves are also concerned. 



In cleaning the muscles of the eyeball, an endeavour should be 

 made to determine the arrangement of the fascia connected with them. 

 A thin, superficial fascia surrounds the various structures contained 

 Avithin the orbit. It is attached to the sphenoid bone around the 

 optic foramen ; while anteriorly it blends with the fibrous tissue of 

 the eyelids. The deep fascia forms sheaths for the various muscles, 

 vessels and nerves, and can be resolved anteriorly into two layers. 

 One of these merges into the fibrous tissue of the eyelids ; the other 

 is attached to the eyeball in the neighbourhood of the corneo-scleral 

 junction. It may be remarked here that the posterior part of the 

 eyeball is surrounded by its own sheath of fascia (fascia bulbi), 

 between which and the eyeball there is a lymph space that com- 

 municates with a prolongation of the subdural space surrounding 

 the optic nerve. 



M. LEVATOR PALPEBR^ SUPERIORIS. — The thin, narrow levator 

 muscle of the upper eyelid is dorsal to the eyeball and immediately 

 underneath the periorbita. Its origin is from the vicinity of the 

 ethmoidal foramen, and its expanded insertion is into the upper 

 eyelid. 



M. OBLIQUUS SUPERIOR. — Of the two oblique muscles of the eyeball 

 the long and narrow superior one arises near the ethmoidal foramen. 

 Passing towards the anterior part of the orbit medial to the medial 

 rectus, it reaches the cartilaginous trochlea underneath the root of 

 the zygomatic process of the frontal bone. After traversing the 

 trochlea, the muscle is reflected, at almost a right angle, outwards 



