THE AXIAL MUSCLES. 



505 



adherent to the muscle-tissue and constitute the fasciae musculares. These fasciae are 

 somewhat thicker in their anterior portions than more posteriorly, and give off pro- 

 longations to neighboring parts. From the fascia of the rectus superior a prolonga- 

 tion passes to join the tendon of the levator palpebrae superioris, and one from the 

 rectus inferior passes to the lower border of the tarsal plate of the lower eyelid, these 

 two recti thus acquiring a certain amount of action upon the eyelids. From the 

 lateral surface of the fascia of the external rectus a rather strong prolongation is 

 given of? which attaches to the orbital surface of the zygomatic arch, forming what 

 has been termed the external check ligament of the eyeball, while from the medial 

 surface of the fascia of the internal rectus a similar, although somewhat laxer, prolon- 

 gation passes to the crest of the lachrymal bone and the reflected portion of the 

 internal palpebral ligament. 



The Movements of the EyebalL — The four recti muscles of the eyeball 

 may be regarded as forming a cone whose apex is at the annulus tendineus communis 



Fig. 517. 



Levator palpebrae superioris 

 Fat 

 Superior rectus \ 

 Capsule of Tenon \ ^^^^\- 



Superior oblique (cut) \ ^^\ . ••l-*^''^ ' 



Fat 

 Optic nerve 



\ 



Septum orbitale 

 Upper fornix of conjunctiva 

 Upper tarsal plate 



Space of Tenon 



Inferior rectus 



Lower tarsal plate 



Space of Tenon 

 Septum orbitale 



Inferior obi 



Diagrammatic sagittal section through orbit, showing relations of fascia to muscles, eyeball, and orbital wall. 



and the base at the insertions of the muscles into the sclera. The line joining the 

 insertions of the muscles is not, however, a circle, but rather a spiral, the insertion 

 of the internal rectus being nearest to and that of the rectus superior farthest from 

 the edge of the cornea. The axis of the cone does not correspond in direction with 

 the antero-posterior axis of the eyeball, but, owing to the divergence of the axes of 

 the two orbits, is inclined to it from within outward at an angle of about 20°. 



It follows from this that during the contraction of either the superior or infe- 

 rior rectus the axis of rotation of the eyeball will not coincide with its transverse 

 axis, but will be inclined to it (Fig. 518), and consequently the action of either of 

 these muscles in directing the pupil upward or downward will be complicated by a 

 certain amount of oblique movement, in the one case inward and in the other case 

 outward. In producing purely upward or downward movements of the pupil the 

 rectus muscles are associated with the oblique ones, the coordination of the inferior 

 oblique with the superior rectus producing a purely upward rotation, while that of the 

 superior oblique with the inferior rectus produces a purely downward movement. 



