188 DISSECTION OF THE DOG 



upper eyelid. (4) The frontal artery (a. frontalis) courses along the lateral 

 border of the superior straight muscle to reach the upper eyelid, where it 

 anastomoses with the superficial temporal artery. (5) Two 'posterior ciliary 

 arteries (aa. ciliares posteriores) accompany the optic nerve into the 

 eyeball. (6) The continuation of the ophthalmic artery, the ethmoidal artery 

 (a. ethmoidalis), traverses the ethmoidal foramen and gains the interior 

 of the cranium, where it divides into meningeal and nasal branches. 



A. centralis retinje. — The small central artery of the retina arises from 

 the anastomotic branch of the ophthalmic which joins the internal carotid 

 artery. The vessel enters the eyeball within the optic nerve and is distributed 

 over the anterior surface of the retina. 



Palpebra tertia. — On making an examination of the undissected orbit, 

 a prominent fold of conjunctiva will be observed medial to the eyeball. This 

 is the membrana nictitans or free edge of the third eyelid, the basis of which 

 is formed by a roughly triangular plate of cartilage (cartilago nictitans). The 

 deeper part of the cartilage is surrounded by a reddish glandular mass (glandula 

 palpebra? tertia? [Harden]). 



Dissection. — Before entering on the dissection of the eyeball (bulbus oculi), 

 it is well that several specimens be secured. Some of these should be 

 examined in the fresh condition ; others should be hardened in a formol 

 solution preparatory to their dissection. From all of them the muscles 

 should be removed. When doing this notice the exit of the vorticose 

 veins about the equator of the eyeball, and the entrance of the optic 

 nerve and the posterior ciliary arteries. 



In the first place the dissector should gain some idea of the general structure 

 of the eyeball by making sections of two specimens. One should be 

 cut in a vertical, antero-posterior direction ; the other in the plane of 

 the equator. 



General Conformation and Structure of the Eyeball. — Although 

 more spherical than in other domestic animals, the eyeball of the dog is not a 

 perfect sphere, as is shown by certain proportions given by Koschel. According 

 to this observer the average transverse, vertical and antero-posterior diameters 

 are in the proportion of 19'7, 18 - 7. and 20 in small dogs, and 24, 23, and 24- 2 

 in large dogs. The excess in the antero-posterior diameter depends upon the 

 bulging of the cornea, a part of a sphere with a shorter radius than that of the 

 sclera by which the greater portion of the curve of the eyeball is determined. 



It is found convenient to employ certain descriptive terms in the consideration 

 of the eyeball. The anterior and posterior poles (polus anterior et polus posterior) 

 are the central points of the anterior- and posterior curvatures respectively. 

 They are joined by an imaginary line, known as the optic axis (axis optica), 



