436 OPTIC NERVE. 



a papilla of grey matter (caruncula mamillaris), embedded in the 

 anterior lobe, and an external root, which may be traced as a white 

 streak along the fissure of Sylvius into the corpus striatum, where it 

 is continuous with some of the fibres of the anterior commissure. 

 The nervous cord formed by the union of these three roots is soft in 

 texture, prismoid in shape, and embedded in a sulcus between two 

 convolutions on the under surface of each anterior lobe of the brain, 

 lying between the pia mater and the arachnoid. As it passes for- 

 wards it increases in breadth and swells at its extremity into an 

 oblong mass of grey and white substance, the bulbus olfactorius, which 

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

 under surface of the bulbus olfactorius are given off the nerves which 

 pass through the cribriform foramina and supply the mucous mem- 

 brane 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 firm, which descend 

 through bony canals in the outer wall of the nares, and are distri- 

 buted 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 



* The isthmus encephali, shewing the thalamus opticus, corpora quadri- 

 gemina, pons Varolii, and medulla oblongata as viewed from the side. 1. The 

 thalamus opticus. 2. The posterior prominence of this body, tuberculum supe- 

 rms posterius or pulvinar. 3. The corpus geniculatum externum. 4. The 

 corpus geniculatum internum. 5. The commencement of the tractus opticus. 

 0. The pineal gland. 7. The nates. 8. The testis of one side. 9. The bra- 



