8 3o THE APPARATUS OF THE SENSES. 



faculty it owes to its reflexion in the cartilaginous loop, as it acts as if its 

 insertion was at the angle it forms there. 



4. SMALL OBLIQUE MUSCLE (obliquus inferior oculi). Much thicker, 

 though very much shorter than the preceding, and almost entirely fleshy, 

 this muscle is placed in a transverse direction on the globe of the eye, being 

 nearly parallel to the reflected portion of the great oblique. It arises in 

 the lachrymal fossa, passes outwards, and terminates in the sclerotic, 

 between the external and inferior recti muscles. 



It is an antagonist of the great oblique, pivoting the eye in a contrary 

 direction. 



It is to be noted that the double rotatory movement executed by the 

 oblique muscles is altogether involuntary, and that it is constantly produced 

 when the animal inclines its head to one side : doubtless to maintain the 

 visual axis always in the identical relations with the same point of the 

 retina. This movement is well seen in Man when the head is brought 

 round to either shoulder : the eye then pivots in the orbit in an inverse 

 direction to that to which the head inclines, so that a mark placed at the 

 upper part of the iris when the head is straight would occupy the same 

 position after the lateral movement. Simultaneous in both eyes, this 

 pivoting is executed by certain muscles in each ; the great oblique for one, 

 the small oblique for the other, according to the direction in which the 

 head is turned. 



(A third, or middle oblique muscle, has been mentioned by the late 

 Professor Strangeways, of the Edinburgh Veterinary School, as some- 

 times, if not always, found between the superior and inferior oblique 

 muscles. It has been described as arising by a fine tendon from a small 

 depression in the upper part of the orbital process of the frontal bone, 

 between the origin of the inferior oblique and the pulley of the superior 

 oblique muscle. This tendon is succeeded by a fusiform fleshy mass, about 

 three lines in diameter and an inch long, imbedded in adipose tissue; 

 it passes obliquely upwards and outwards on the external face of the 

 rectus muscle, and terminates in a thin flat tendon which accompanies the 

 upper belly of the superior oblique for a short distance, and becomes con- 

 founded with the tendon of that muscle as it runs beneath the superior rectus. 

 It is supposed to be an accessory of the superior oblique, and to regulate 

 and facilitate the gliding of that muscle through the acute angle formed by 

 its pulley.) 



PROTECTIVE ORGANS OF THE EYE. 



1. The Eyelids. (Figs. 383, 391.) 



The surface of the eye is covered and protected in front, by two movable 

 membranous curtains the eyelids (palpebrce) : one superior, the other inferior. 



Attached to the circumference of the orbit by their external border, the 

 eyelids have a convex external face formed by the skin, and a concave 

 internal face, moulded on the anterior surface of the eye, and lined by the 

 conjunctiva which is reflected above and below on the eyeball : the duplica- 

 tures constituting the superior and the inferior conjunctival (or palpebral) 

 sinuses. 



Each lid has also a free border opposed to that of its fellow, with which 

 it unites at an angle by its extremities, so as to form two commissures (or 

 canthi). This border is slightly bevelled on the inner side, and shows a 

 series of small openings the excretory orifices of the Meibomian glands ; as 



