THE HEAD AND NECK 1149 



Inferior Rectus — Origin. — The lower part of the fibrous ring in 

 relation to the optic foramen. 



Insertion. — By a thin expanded tendon into the lower surface of 

 the sclerotic about three or four lines from the margin of the cornea. 



Nerve-supply. — ^The lower division of the third cranial nerve, the 

 branch from which enters the deep or ocular surface of the muscle. 



The muscle is flattened from above downwards, and its direction 

 is forwards and slightly outw^ards. 



Action. — ^To depress the eyeball, so as to direct the cornea down- 

 wards. The muscle imparts a certain amount of inward movement 

 to the eyeball, accompanied by slight rotation. 



Relations — Inferior. — A part of the inferior oblique. 



External Rectus — Origin. — ^This muscle arises by two heads. 



Inferior Head. — (i) The lower part of the fibrous ring in relation 

 to the optic foramen, where it is contiguous to the inferior rectus ; 

 and (2) the prominent spine on the lower margin of the sphenoidal 

 fissure near its inner end. 



Superior Head. — The outer portion of the upper part of the fibrous 

 ring in relation to the optic foramen, where it is above the sphenoidal 

 fissure, external to the optic foramen, and contiguous to the superior 

 rectus. The two heads are connected by a tendinous band which 

 arches over the sphenoidal fissure, and gives origin to some of the 

 fibres of the inferior head of the muscle. 



Insertion. — ^The outer surface of the sclerotic from three to four 

 lines from the margin of the cornea. 



Nerve-supply. — ^The sixth cranial ner\'e, the branches of which 

 enter the deep or ocular surface of the muscle. 



The muscle is flattened from without inwards, and its direction 

 is forwards and outwards. 



Action. — ^To abduct the eyeball, so as to direct the cornea out- 

 wards. 



Relations. — ^The following structures pass between the two heads 

 of the muscle, in order from above downwards : (i) the upper 

 division of the third nerve ; {2) the nasal nerve ; (3) the lower divi- 

 sion of the third nerve ; (4) the sixth nerve ; and (5) the ophthalmic 

 vein, or veins. 



Common Tendon of the Recti Muscles. — ^This tendon takes 

 the form of a fibrous ring, which is attached to the upper, 

 inner, and lower margins of the optic foramen. It then crosses 

 transversely the inner portion of the sphenoidal fissure to a promi- 

 nent spine on the lower margin of that fissure near its inner end. 

 From this spine it recrosses the sphenoidal fissure in a vertical 

 direction near its centre, and finally reaches the upper part of the 

 optic foramen. This latter portion gives origin to some fibres of 

 the inferior head of the external rectus. The fibrous ring presents 

 an upper and a lower part. The upper part is prolonged forwards 

 into the superior tendon of Lockwood, which gives origin from within 

 outwards to part of the internal rectus, the superior rectus, and 

 the superior head of the external rectus. The lower part is also 



