146 OUTLINES OF EQUINE ANATOMY. 



smallest nerves at tlie base of the brain, and wind round 

 the outer surface of the crura cerebri, puttinsr in their 

 appearance just in front of the pons Varolii. They gene- 

 rally are found emerging from the cranium through 

 foramen lacerum orbitale (and not through foramen 

 pathetici as their name might lead us to expect, and which 

 only gives passage to small vessels). These nerves run 

 direct to the superior oblique muscles to which they give 

 motor power. 



Abducens or sixth pair of nerves arise from the under 

 surface of the medulla oblongata, just behind the pons 

 Varolii, on either side of the inferior longitudinal fissure. 

 Each nerve passes through foramen lacerum orbitale in 

 company with the third, fourth, and ophthalmic division of 

 the fifth nerves, and the ophthalmic artery of the side to 

 which it belongs, and on gaining the orbit runs to supply 

 the abducens muscle. 



The eyeball or globe of the eye is a spherical body, 

 slightly flattened from before backwards. The posterior 

 three fourths is formed by a white fibrous coat, the sclerotic, 

 its anterior part by a transparent layer, the cornea. To its 

 postero-inferior and internal part runs the optic nerve, 

 enclosed in its dense sheath of dura mater. The eyeball 

 presents three coats, three cavities, and three humours, 

 with accessory parts. The sclerotic is a fibrous coat, which 

 is externally situated at the posterior three fourths of the 

 eyeball. It is thickest posteriorly, and grows thinner to 

 about the centre, when again it commences to grow thick, 

 and anteriorly the cornea fits into it " like a watch-glass 

 into its case. Around the opening of the optic nerve it 

 becomes attached to its sheath, which becomes inflected 

 inwards forming a plate, lamina cribrosa, through numerous 

 perforations in which the fibres of the nerve pass. Through 

 the largest of these perforations, termed the porus opticus, 

 the arteria centralis retinae passes. Numerous small fora- 

 mina, through which the ciliary nerves and vessels pass, 

 are to be seen on the surface of the sclerotic. Anteriorly 

 it is directly continuous with the cornea, so that we cannot 

 distinguish between them with the microscope, though 

 the transparency of the cornea is sufficiently distinctive, 

 and seems to be due to fluid contained between its laminae, 

 since opacity is caused by pressure. It is a segment of a 

 smaller sphere than the sclerotic, which overlaps it most 



