RECTI MUSCLES. 57 



the nose cavity through the openings in the cribriform plate of the 

 ethmoid. 



The anterior branch (internal nasal) accompanies the nasal nerve to 

 the cavity of the nose (Section XIV.), and gives likewise meningeal off- 

 sets to the dura mater, and the fore part of the falx cerebri. 



The palpebral branches, one for each eyelid, generally arise together 

 opposite the pulley of the superior oblique muscle, and then separate from 

 one another. The arches they form have been dissected with the eyelids 

 (P- 45). 



The frontal branch turns round the margin of the orbit, and is distrib- 

 uted on the forehead (p. 21). 



The ophthalmic vein corresponds in its course and most of its branches 

 with the artery of the same name. It begins at the inner angle of the 

 orbit, where it joins the facial vein, and receives tributary branches in its 

 progress to the back of the cavity. Posteriorly it leaves the artery, and 

 escapes from the orbit by the sphenoidal fissure between the heads of the 

 external rectus, to end in the cavernous sinus. 



The OPTIC NERVE in the orbit extends from the optic foramen to the 

 back of the eyeball. As the nerve leaves the foramen it is surrounded by 

 the recti muscles ; and beyond that spot the ciliary arteries and nerves 

 entwine around it. It terminates in the retinal expansion of the eye. 



Dissection (fig. 12). Take away the ophthalmic artery, and divide the 

 optic nerve about its middle, together with the small ciliary vessels and 

 nerves. Turn forwards the eyeball, and fasten it in that position with 

 hooks. On removing some fat the three recti muscles inner, inferior, 

 and outer, will appear ; and lying on the two first, are the offsets of the 

 lower branch of the third nerve. 



The lower branch of the third nerve (fig. 12) supplies three muscles in 

 the orbit. Whilst entering this space between the heads of the external 

 rectus, it lies below the nasal, and rather above the sixth nerve. Almost 

 immediately afterwards the nerve divides into three branches. One ( 5 ) 

 enters the internal rectus ; another ( 4 ) the inferior rectus ; and the third 

 ( 3 ), the longest and most external, is continued forwards to the inferior 

 oblique muscle which it pierces at the hinder border. 



Soon after its origin the last branch communicates with the lenticular 

 ganglion, forming the short root (fig. 11, 3 ) of that body; and it furnishes 

 two or more filaments to the inferior rectus. 



The SIXTH NERVE (fig. 12, 2 ) lies below the other nerves, and above 

 the ophthalmic vein, in the interval between the heads of the external 

 rectus. In the orbit it is distributed to the external rectus muscle. 



RECTI MUSCLES. The internal D, inferior c, and external rectus K 

 (fig. 12) are placed with reference to the eyeball as their names express. 

 They arise posteriorly from the circumference of the optic foramen by a 

 common attachment, which partly surrounds the optic nerve. But the 

 external rectus differs from the others in having two heads : The upper 

 joins the superior rectus in the common origin. The lower and larger head 

 blends on the one side with the inferior rectus in the common origin, and 

 is attached in addition to a bony point on the lower border of the sphenoidal 

 fissure, near the inner end ; whilst some of its muscular fibres are con- 

 nected with a tendinous band between the heads. All the muscles are 

 directed forwards, but the lower obliquely outwards, and have a tendinous 

 insertion into the ball of the eye about a quarter of an inch from th< j 

 cornea, and in front of the transverse diameter of the ball. 



