394 



OPTIC NERVE. 



Fig. 177.* 



into an obiong mass of grey and white substance, the bulbus olf act onus, 

 which rests upon the cribriform lamella of the ethmoid bone. From the 



under surface of the bulbous olfacto- 

 ries are given off the nerves which 

 pass through the cribriform foramina 

 and supply the mucous membrane 

 of the nares; they are arranged into 

 two groups, an inner group, reddish 

 in colour and soft, which spread out 

 upon the septum narium, and an 

 outer group, whiter and more linn, 

 which descend through the bony ca- 

 nals in the outer wall of the nares, 

 and are distributed upon the superior 

 and middle turbinated bones. 



SECOND PAIR. OPTIC. The optic 

 nerve, a nerve of large size, arises 

 from the corpora geniculata on the 

 posterior and inferior aspect of the 

 thalamus opticus and from the nates. 

 Proceeding from this origin it winds 

 around the crus cerebri as a flattened 

 band, under the name of tractus opticus, and joins with its fellow in front 

 of the tuber cinereum to form the ovtic commissure (chiasma). The tractus 



opticus is united with the crus 

 cerebri and tuber cinereum, and 

 is covered in by the pia mater ; 

 the commissure is also connected 

 with the tuber cinereum, from 

 which it receives fibres, and the 

 nerve beyond the commissure 

 diverges from its fellow, becomes 

 rounded in form, and is enclosed 

 in a sheath derived from the 

 arachnoid. In passing through 

 the optic foramen the optic nerve 

 receives a sheath from the dura 

 mater, which splits at this point 

 into two layers ; one, which be- 

 comes the periosteum of the 



* A view of the 2d pair or optic, and the origins of seven other pairs. 1, 1. Globe 

 of the eye, the one on the left hand is perfect, but that on the right has the sclerotic ;md 

 choroid removed to show the retina. 2. The chiasm of the optic nerves. 3. The cor- 

 pora albicantia. 4. The infundibulum. 5. The pons Varolii. 6. The medulla ob- 

 longata. The figure is on the right corpus pyramidale. 7. The 3d pair, motores oculi. 

 8. 4th pair, pathetici. 9. 5th pair, trigemini. 10th. 6th pair, abducentes. 11. 7th pair, 

 auditory and facial. 12th. 8th pair, pneumogastric, spinal accessory, and glosso-pha- 

 ryngral. 13. 9th pair, hypoglossal. 



+ The isthmus encephali, showing the thalamus opticus, corpora qnadrigemina. pons 

 Varolii, and medulla oblongata, as viewed from the side. 1. The thalamus opticus. 



2. The posterior prominence of this body, tuberculum superius posterius or pulvinar. 



3. The corpus geniculatum externum. 4. The corpus geniculatum internum. 5. The 

 commencement of the tractus opticus. 6. The pineal gland. 7. The nates. 8. The 



I 



Fig. 178.t 



