THE CRANIAL NERVES THE OPTIC NERVE 793 



the cranial cavity through the foramen magnum, and divides at the interpeduncular 

 fossa into the two posterior cerebral arteries. These concur with branches of the 

 internal carotid arteries in fo'rming the remarkable arterial circle on the base of the 

 cerebrum. From these emanate basal arteries which supply in general the brain 

 stem and basal ganglion, and cortical arteries which in general run superficially and 

 supply the cortical substance and medullary white substance. 



The veins enter the sinuses of the dura mater, and in the main are not satel- 

 lites of the arteries. The sinuses communicate with the ophthalmic, dorsal and 

 ventral cerebral, and spinal veins. 



The Cranial Nerves 



The cranial or cerebral nerves (Nn. cerebrales) comprise twelve pairs which 

 are designated from before backward numerically and by name. Their number, 

 names, and functional characters are given in the subjoined table: 



I. Olfactory Sensory (Smell) 



II. Optic Sepsory (Sight) 



III. Oculomotor Motor 



IV. Trochlear Motor 



V. Trigeminal Mixed 



VI. Abducent Motor 



VII. Facial Mixed 



VIII. Acoustic Sensory (Hearing and Equilibration) 



IX. Glosso-pharyngeal Mixed 



X. Vagus Mixed 



XI. Spinal accessory Motor 



XII. Hypoglossal Motor 



THE OLFACTORY NERVE 



The olfactory nerve (N. olfactorius) is peculiar in that its fibers are not aggre- 

 gated to form a trunk, but are connected in small bundles with the olfactory bulb. 

 They are non-medullated, and are the central processes of the olfactory cells which 

 are situated in the olfactory region of the mucous membrane of the nasal cavity. 

 This region is distinguished by its brown color and comprises the posterior part of 

 the lateral mass of the ethmoid, a small adjacent area of the dorsal turbinate, and 

 the corresponding surface of the septum nasi. The nerve-bundles are enclosed in 

 sheaths derived from the membranes of the brain and pass through the foramina of 

 the cribriform plate to join the convex surface of the olfactory bulb. Some 

 fibers come from the vomero-nasal organ. 



THE OPTIC NERVE 



The optic nerve (N. opticus) is composed of fibers which are the central proc- 

 esses of the ganglion cells of the retina. The fibers converge within the eyeball to 

 the optic papilla, where they are collected into a round trunk, the optic nerve. The 

 nerve thus formed pierces the chorioid and sclera, emerges from the posterior part 

 of the eyeball, and passes backward and medially to the optic foramen. After 

 traversing the latter it decussates with its fellow of the opposite side to form the 

 optic chiasma. In the orbit the nerve is slightly flexuous and is embedded in the 

 fat behind the eyeball and surrounded by the retractor oculi muscle. Its intraos- 

 seous part is an inch or more (ca. 3 cm.) long. The sheath of the nerve is formed by 

 prolongations of the membranes of the brain, and includes continuations of the 

 subdural and subarachnoid spaces. 



The greater part of the fibers of the optic nerve cross in the chiasma to the tract of the oppo- 

 site side. In the tract the fibers proceed to (1) the medial geniculate body, (2) the posterior 



