144 OUTLINES or EQUINE ANATOMY. 



This fat is found even in tlie most emaciated subjects. 

 "We have already noticed the levator palpebrae superioris 

 internus ; by incision through the periorbital we may now 

 expose it : a thin layer of muscle which arises from the 

 upper margin of the sphenoicleal hiatus and runs to the 

 upper eyelid, on which it is enabled to act by gaining 

 leverage over the upper surface of the eyeball. The 

 periorbitale is a dense fibrous membrane which lines the 

 orbital cavity, forming the sole protection to the contained 

 structures at the postero-inferior and suj)erior parts, which 

 in the human subject are bony. Anteriorly it is attached 

 to the margin of the orbit, posteriorly around the sphe- 

 noideal hiatus ; it encloses the eyeball and its muscles. 

 From around this hiatus other muscles arise and run to 

 the eyeball. 



Retractor oculi, consisting of four distinct portions, 

 arises from directly around it, and after passing in a 

 forward direction in contact with the optic nerve becomes 

 inserted into the posterior part of the extei'nal surface of 

 the sclerotic coat. The four 



RECTI — externus or abductor, internus or adductor, 

 superior or levator, and inferior or depressor — arise from 

 the external, internal, superior, and inferior parts of the 

 spheiioideal hiatus respectively, and 2:>ass to the anterior 

 part of the sclerotic, and form aponeurotic expansions 

 which unite at their margins respectively, forming thus a 

 continuous layer, situated around the outer margin of the 

 cornea, covered anteriorly by conjunctiva, tunica albuginea 

 (forming the " white of the eye "). 



Superior oblique muscle arises from just above the 

 straight muscles at the upper part of the sphenoideal 

 hiatus. It then runs to the root of the orbital process of 

 the frontal bone, where it passes through a fibro- cartilagi- 

 nous loop. From this it runs to the upper part of the eye- 

 ball, forms an aponeurotic tendon which runs beneath the 

 tendon of levator oculi, and becomes inserted into the 

 anterior part of the sclerotic between the last-mentioned 

 muscle and abductor oculi. Sirangeways has noticed an 

 accessory muscle which arises near the fibrous loop and he- 

 comes blended with this; we have frequently verified this 

 observation. 



Inferior oblique muscle arises from the orbital plate of 

 the lachrymal bone, near the lachrymal foramen, runs 



