738 THE SENSE ORGANS AND SKIN OF THE HORSE 



little ventrally about to a transverse plane through the first cheelc tooth and lies in the groove 

 above the inferior turbinal crest. Beyond this the duct inclines upward and widens very con- 

 siderably, crosses the nasal process of the premaxilla obliquely, and contracts at its termination. 

 The mucous membrane may present valvular folds, the most distinct of which is situated at the 

 origin. 



THE PERIORBITA 



The periorbita or ocular sheath is a conical fil)rous membrane which incloses 

 the eyeball with its muscles, vessels, nerves, etc. Its apex is attached around the 

 optic and lacerated foramina, and its base is in part attached to the bony rim of 

 the orbit, in part continuous with the fil)rous layer of the lids. Its inner part, 

 which is in contact with the orbital wall, is thin; incorporated with it beneath the 

 root of the supraorbital process is the bar of cartilage around which the superior 

 oblique muscle is reflected. The outer part is thicker, and is strengthened by an 

 elastic band which is attached to the pterygoid crest and furnishes origin to the 

 thin unstriped orl)ital muscle. A quantity of fat (Corpus adiposum extraorbitale) 

 lies about the periorbita, and within it is the intraorbital adipose tissue (Corpus 

 adiposum intraorbitale) which fills the interstices between the eyeball, muscles, etc. 



THE ORBITAL FASCIA AND OCULAR MUSCLES (Figs. 438, 439, 555) 



The straight muscles of the eyeball and the o])lique muscles in part are inclosed 

 in fibrous sheaths (Fasciae musculares), formed by superficial and deep layers of 

 fascia, which are united by intermuscular septa in the interstices between the 

 muscles. The superficial fascia is thin; it lilends in front with the fibrous layer of 

 the eyelids and is attached l)ehind around the optic foramen. The deep fascia 

 consists anteriorly of two layers, one of which is continuous with the fibrous tissue 

 of the lids, while the other is attached at the corneo-scleral junction. 



The posterior part of the eyeball is covered by the bulbar fascia or capsule of 

 Tenon (Fascia bulbi), so that between them a lymph space (Spatium interfasciale) 

 is inclosed which communicates with the sul)dural space along the course of the 

 optic nerve. 



The levator palpebrae superioris muscle ^ is a thin l)and about half an inch in 

 width which lies above the rectus superior. It is narrow at its origin above and 

 behind the ethmoidal foramen and ends l)y an expanded tendon in the upper lid. 

 Its action is to raise the upper lid. 



The recti oculi (Mm. recti bulbi) are four in number and are designated ac- 

 cording to their positions as superior, inferior, internal, and external. They are all 

 V)and-like, arise close tog(^ther around the ()])lic foramen, and diverge as they pass 

 forward to the eyeball. On rcuching the latter they end in thin tendons which are 

 inserted into the sclera in front of the equator of the eyeball. 



The retractor oculi (M. retractor bulbi) surrounds the optic nerve, and is 

 incompletely divided into four parts which alt(Tnate with the recti. They arise 

 around the optic nerv(^ and are inserted into th(> sclera behind the recti. 



The obliquus oculi superior (M. ol)liquus })ulbi dorsalis s. superior) is the longest 

 and narrowest of tiie ocular muscles. It arises near the ethmoidal foramen and 

 passes forward internal to the rectus internus. Under the root of the supraorbital 

 process it is reflected almost at a right angle aroimd a cartilaginous pulley (trochlea), 

 which is attached to the anterior ]iart of the inner wall of the orljit, a bursa Ijeing 

 interposed here. The muscle is then directed outward and somewhat forward, and 

 ends in a thin tendon which passes between the rectus superior and the eyel)all, 

 and is inserted into the sclera between the superior and external recti, about half 

 an inch behind the margin of the cornea. 



The obliquus oculi inferior (M. obliquus bulbi ventralis s. inferior) is wide and 

 * This belongs to the upper eyelid, Imt is described here on account of its position. 



