DISSECTION OF THE DOG 187 



Mm. recti oculi. — There are four straight muscles of the eyeball, namely, 

 superior, inferior, medial, and lateral. They all arise in the neighbourhood of 

 the optic foramen and are inserted by flattened tendons into the fore part of the 

 eyeball. 



N. octtlo-motoritts. — The oculo-motor or third cerebral nerve enters the 

 orbit at its apex and supplies all the muscles of the eyeball with the exception 

 of the superior oblique and the lateral rectus. 



Two main divisions of the nerve may be distinguished. Of these the 

 upper (ramus superior) supplies branches to the retractor and superior straight 

 muscles of the eyeball and the levator of the upper eyelid. The lower division 

 (ramus inferior) is larger and travels between the inferior and lateral straight 

 muscles to end in the inferior oblique. In addition it furnishes nerves to the 

 inferior and medial straight muscles and to the retractor of the eyeball. 



The small ciliary ganglion (ganglion ciliare) lies between the origins of the 

 nerves to the inferior oblique and inferior straight muscles. Two or three 

 short ciliary nerves leave it to accompany the optic nerve into the eyeball. 



N. abducens. — The abducent or sixth cerebral nerve will be found running 

 along the border of the lateral straight muscle, into which it disappears at a 

 point about the middle of the length of the muscle. 



M. obliquus inferior. — The inferior oblique muscle is ventral and lateral 

 to the eyeball, and differs from the other muscles in not having its origin in the 

 vicinity of the optic foramen. The origin is close to the lachrymal fossa, from 

 which place the muscle passes under cover of the lateral rectus to be inserted 

 into the sclera. 



M. retractor oculi. — Composed of four distinct parts (superior, inferior, 

 lateral, and medial) which all arise about the optic foramen, the retractor 

 of the eyeball lies and is inserted into the sclera within the four straight 

 muscles. 



N. opticus. — The thick, rounded, cord-like optic or second cerebral nerve 

 forms a conspicuous object in its transit from the optic foramen to the back 

 of the eyeball. 



A. ophthalmica. — As has been seen, immediately the internal maxillary 

 artery emerges from the alar canal it gives off the ophthalmic artery. This 

 pursues a somewhat circuitous course round the structures at the apex of the 

 orbit in order to gain the ethmoidal foramen, which it enters as the ethmoidal 

 artery. 



The branches of the ophthalmic artery are as follows : (1) Close to its 

 origin an anastomotic branch leaves it to enter the orbital fissure and join the 

 internal carotid artery within the cranium. (2) There are generally two main 

 muscular branches (rami musculares) which supply not only the ocular 

 muscles but also the fat within the orbit, the periorbita, and the lachrymal 

 gland. (3) The lachrymal artery (a. lacrimalis) is a small vessel accompanying 

 the nerve of the same name and terminating in the lachrymal gland and the 



