TEE ACCESSORY ORGANS OF VISION. 939 



elastic and inelastic fibres. (Uiistriped muscular fibres have also been described 

 as existing in this orbital periosteum.) 



Thus completed, the orbital cavity has the form of a regular hollow cone, 

 ojien at its base, and closed at the apex, which corresponds to the orbital hiatus. 



In the ordinary position of the head, the opening of this cone is directed 

 forwards, dowinvards, and outwards. 



Independently of the globe of the eye, the orbital cavity lodges the muscles 

 that move it, the membrana nictitans, and the lachrymal gland. 



(Unstriped muscular fibres have been found in this ocular sheath in Sheep 

 and other animals, also in Man.) 



Muscles of the Globe of the Eye (Fig. 510). 



These are seven in number : five termed rprfi muscles, and distinguished as 

 posterior, superior, inferior, externaJ, and ititeinaJ ; two named oblique — a large 

 and small. 



(Preparation.— Detsich the eyelids from the margin of thp orbit, cutting away the lower, 

 but leaving the upper. Saw through the zygomatic process of the temporal bone, in front of 

 the temporo-maxillaiy articulation, also through the temporal process of the malar, and the 

 base of the orbital process of the frontal bone ; remove the excised piece of bone, and the 

 temporal fossa and ocular sheath are exposed. Cutting through the latter, the muscles of 

 the eye are seen disposed in a conical manner around tiie globe ; dissect away the fat lodged 

 among them, in order to isolate them.) 



1. Posterior Rectus or Suspensory Muscle {retractor oculi, retractor 

 hulhi). — This muscle completely envelops the extra-cranial portion of the optic 

 nerve, being a muscular sheath resembling in shape the fibrous lining of the 

 orbit. Its fibres are disposed longitudinally, arise around the optic foramen, and 

 are inserted into the posterior part of the external face of the sclerotic. It is 

 always more or less fasciculated, and may be frequently separated into four 

 portions — superior, inferior, e.Kternal, and internal. 



In contracting, it draws the globe towards the back of the orbit. The 

 physiological result of this movement will be noticed hereafter. 



2. Superior, Inferior, External, and Internal Recti Muscles. — These 

 four muscles are placed longitudinally on the preceding, and repeat, on a large 

 scale, the disposition of its four bundles. As their borders are in contact, they 

 constitute a fleshy sheath around it, analogous to that which it forms around the 

 optic nerve. Exactly resembling each other, these four muscles compose so 

 natural a group, that they may be described together. Each is a flat band, 

 formed of parallel fibres, firmly attached by its posterior extremity to the back 

 of the sheath, and to the interior of the subsphenoidal canal ; anteriorly, it is 

 inserted by a thin aponeurosis into the sclerotic, at the margin of the cornea. 

 Isolated from one another, and from the retractor by the mass of fat belonging 

 to the membrana nictitans, these small muscles are related, externally, to the 

 ocular sheath. 



There is nothing particular to be noted regarding them, their position being 

 sufficiently indicated by their names. Their function is to bring the pupillary 

 opening into contact with the rays of light, by inclining the cornea towards them, 

 either upwards, downwards, inwards, or outwards ; or into intermediate positions, 

 which happens when two adjacent muscles — the inferior and external rectus, for 

 instance— combine their action at the same moment. 



