1042 



ORGANS OF SPECIAL SENSE 



being therefore slightly convex backwards. The nerve-trunk here quickly becomes 

 reduced to one-half its former diameter, the fibres losing their medullary sheath, 

 and being continued henceforward as mere axis-cylinders. Apart from the conse- 

 quent loss of bulk, this histologieal change may be readily recognised macroscopi- 

 cally in a longitudinal section of the nerve, its aspect here changing from opaque 

 white to semi-translucent grey. The part of the nerve within the lamina cribrosa 

 has already been seen in our ophthahnoscopic examination of the living eye. 



The optic nerve is mainly nourished by fine vessels derived from those of the 

 pial sheath, which run into the substance of the nerve in the processes above men- 

 tioned. In front of the entrance of the central retinal artery this vessel aids to 

 some extent in the blood-supply of the axial part of the nerve. 



THE BLOOD-VESSELS AND NERVES OF THE ORBIT 



As these structures will be more particularly described in other sections of this 

 work, a very short general account will suffice here. 



Arteries. The main blood-supply is afforded by the ophthalmic artery, a 

 branch of the internal carotid, which gains the orbit through the optic canal, where 

 it lies beneath and to the temporal side of the nerve. On entering the orbit it 

 ascends, and passes obliquely over the optic nerve to the inner wall of the orbit; 

 in this early part of its course it gives off most of its branches, which vary much 

 in their manner of origin and also in their course. The arteries of the orbit are 



FIG. 741. THE BLOOD-VESSELS OF THE LEFT ORBIT, VIEWED FROM ABOVE. 



Supraorbitnl artery 

 LACHRYMAL GLAND 

 Superior rectus 



EYEBALL 



External rectue 

 Lachrymal artery 



Superior reotus, eut 

 ic vein 



Superior ophthalmic rein 



OPTIC NER VE 

 Common ophthalmic rein 



Commencement of superior 



tijiht/ulllltic ii'in 



Reflected tendon of superior obliqu 



l in if urfi'i'y 



-Shr Anterior etli itioi'htl artrr'i 



r< tit ri'ii' ftliinoiihtl (trtt't'if 

 Ciliary ttrfri-i>.-t 



Levator palpebrse, cut 



Ligament ot'Zinn 

 Ophthalmic artery 



OPTIC COM MISS CUE 



Internal 



artery 



remarkable for their tortuous course, for their delicate walls, and for their loose 

 attachment to the surrounding tissues. The ophthalmic artery gives off special 

 branches in the orbit to the lachrymal gland, the muscles, the retina (through the 

 optic nerve), and the eyeball, as well as to the meninges, the ethmoidal cells, and the 

 nasal mucous membrane. Twigs from all the different branches go to supply the 

 fat, fasciae, and ordinary nerves of the orbit. Branches which leave the orbit ante- 

 riorly ramify on the forehead and nose, and also go to the supply of the eyelids 

 and the tear-passages. The ophthalmic artery has many anastomoses with branches 

 of the external carotid. The contents of the orbit are also supplied in part by the 

 infraorbital artery, a branch of the internal maxillary; in particular this artery 



