150 OCULAR GROUP. 



border of the internal rectus, and the vessels and nerves in rela- 

 tion with that border. 



The obliqiLus inferior, a thin and narrow muscle, arises from the 

 inner margin of the superior maxillary bone, immediately external 

 to the lachrymal groove, and passes beneath the inferior rectus, to 

 be inserted into the outer and posterior part of the eyeball, at about 

 two lines from the entrance of the optic nerve. 



Relations. By its superior surface with the inferior rectus muscle 

 and with the eyeball ; and by the inferior surface with the perios- 

 teum of the floor of the orbit and the external rectus muscle. 



According to Mr. Ferrall* the muscles of the orbit are separated 

 from the globe of the eyeball and from the structures immediately 

 surrounding the optic nerve, by a distinct fascia, which is continuous 

 with the broad tarsal ligament and with the tarsal cartilages. This 

 fascia the author terms, the tunica vaginalis oculi ;\ it is pierced 

 anteriorly for the passage of the six orbital muscles, by six openings 

 through which the tendons of the muscles play as through pulleys. 

 The use assigned to it by Mr. Ferrall is to protect the eyeball from 

 the pressure of its muscles during their action. By means of this 

 structure the recti muscles are enabled to impress a rotatory move- 

 ment upon the eyeball ; and in animals provided with a retractor 

 muscle, they also act as antagonists to its action. 



Actions. The levator palpebrse raises the upper eyelid. The four 

 recti, acting singly, pull the eyeball in the four directions of upwards, 

 downwards, inwards, and outwards. Acting by pairs, they carry 

 the eyeball in the diagonal of these directions, viz. upwards and 

 inwards, upwards and outwards, downwards and inwards, or 

 downwards and outwards. Acting altogether, they directly retract 

 the globe within the orbit. The superior oblique muscle, acting 

 alone, rolls the globe inwards and forwards, and carries the pupil 

 outwards and downwards to the lower and outer angle of the orbit. 

 The inferior oblique, acting alone, rolls the globe outwards and 

 backwards, and carries the pupil outwards and upwards to the 

 upper and outer angle of the eye. Both muscles acting together, 

 draw the eyeball forwards, and give the pupil that slight degree of 

 eversion which enables it to admit the largest field of vision. 



4. Nasal Group. Pyramidalis nasi,J 

 Compressor nasi. 



/The pyramidalis nasi is a small pyramidal slip of muscular fibres 

 sent downwards upon the nose by the occipito-frontalis. It is 

 inserted into the tendinous expansion of the compressores nasi. 



Relations. By its upper surface with the integument; by its 

 under surface with the periosteum of the nasal bone. Its outer 



* In a paper read before the Royal Society, on the 10th of June, 1841. 

 t This fascia was first described by Mr. Dalrymple in his work on the " Anatomy 

 of the Human Eye." 1834. 



t This is described by Homer as one of the insertions of the occipito-frontalis. G. 



