OPHTHALMIC NERVE. 51 



Dissection. The periosteum is next to be divided along the middle of 

 the orbit, and to be taken away. After the removal of a little fat, the 

 following nerves, vessels, and museles come into view ; but it is not need- 

 ful to remove much of the fat in this stage of the dissection. 



The frontal nerve and the supra-orbital vessels lie in the centre ; the 

 lachrymal nerve and vessels close to the outer wall ; and the small fourth 

 nerve at the back of the orbit : all these nerves enter the cavity above the 

 muscles. The superior oblique muscle is recognized by the fourth nerve 

 entering it : the levator palpebroR and superior rectus lie beneath the 

 frontal nerve ; and the external rectus is partly seen below the lachrymal 

 nerve. In the outer part of the orbit, near the front, is the lachrymal 

 gland. 



The frontal and lachrymal nerves should be followed forwards to their 

 exit from the orbit, and backwards with the fourth nerve, through the 

 sphenoidal fissure, to the w r all of the cavernous sinus. In tracing them 

 back, it will be expedient to remove the projecting clinoid process, should 

 this still remain ; and some care will be required to follow the lachrymal 

 nerve to its commencement. 



Contents of the orbit. The eyeball and the lachrymal gland, and a great 

 quantity of granular fat, are lodged in the orbit. Connected with the eye 

 are six muscles four straight and two oblique ; arid there is also an ele- 

 vator of the upper eyelid in the cavity. 



The nerves in this small space are numerous, viz., the second, third, 

 fourth, ophthalmic of the fifth, and the sixth nerve, together with the small 

 temporo-malar branch of the superior maxillary nerve, and offsets of the 

 sympathetic ; their general distribution is as follows : The second nerve 

 penetrates the eyeball ; the third is furnished to all the muscles of the 

 cavity but two ; the fourth enters the superior oblique (one of the two ex- 

 cepted) ; and the sixth is spent in the external rectus muscle. The fifth 

 nerve supplies some filaments to the eyeball with the sympathetic, but the 

 greater number of its branches pass through the orbital cavity to the face. 

 The ophthalmic vessels are also contained in the orbit. 



The lachrymal gland (fig. 10, F) secretes the tears, and is situate in 

 the hollow on the inner side of the external angular process of the frontal 

 bone. It is of a lengthened form, something like an almond, and lies 

 across the eye. From its anterior part a thin accessory piece projects be- 

 neath the upper eyelid. The upper surface is convex, and in contact with 

 the periosteum, to which it is connected by fibrous bands that constitute a 

 ligament for the gland ; the lower surface rests on the eyeball and the 

 external rectus muscle. 



In structure the lachrymal resembles the salivary glands ; and its very 

 fine ducts, from eight to twelve in number, open by as many apertures in 

 a semicircular line on the inner aspect of the upper eyelid towards the 

 outer canthus. 



The FOURTH NERVE (fig. 10, *) is the most internal of the three nerves 

 entering the orbit above the muscles. After reaching this space, it is 

 directed inwards to the superior oblique muscle, which it pierces at the 

 orbital surface, contrary to the general mode of distribution of the nerves 

 on the ocular surface of the muscles. 



The OPHTHALMIC TRUNK of the fifth nerve (fig. 10, p. 52), as it ap- 

 proaches the sphenoidal fissure, furnishes from its inner side the nasal 

 branch, and then divides into the frontal and lachrymal branches ; the 



