792 THE NERVOUS SYSTEM. 



THE CRANIAL NERVES. 



The cranial nerves arise from some part of the cerebro-spinal centre, and 

 are transmitted through foramina in the base of the cranium. They have been 

 named numerically, according to the order in which they pass through the dura 

 mater lining the base of the skull. Other names are also given to them, derived 

 from the parts to which they are distributed or from their functions. Taken in 

 their order, from before backward, they are as follows : 



1st. Olfactory. 7th. Facial (Portio dura). 



2d. Optic. 8th. Auditory (Portio mollis). 



3d. Motor oculi. 9th. Glosso-pharyngeal. 



4th. Pathetic. 10th. Pneumogastric (Par vagum). 



5th. Trifacial (Trigeminus). llth. Spinal accessory. 



6th. Abducent. 12th. Hypoglossal. 



All the cranial nerves are connected to some part of the surface of the brain. 

 This is termed their superficial or apparent origin. But their fibres may, in all 

 cases, be traced deeply into the substance of the brain to some special centre of 

 gray matter, termed a nucleus. This is called their deep or real origin. The 

 nerves, after emerging from the brain at their apparent origin, pass through 

 foramina or tubular prolongations in the dura mater, leave the skull through 

 foramina in its base, and pass to their final distribution. 



First Nerve (Fig. 473, page 783). 



The First Cranial or Olfactory Nerves, the special nerves of the sense of smell, 

 are about twenty in number. They are given off from the under surface of the 

 olfactory bulb, an oval mass of a grayish color, which rests on the cribriform 

 plate of the ethmoid bone, and forms the anterior expanded extremity of a slender 

 process of brain-substance, named the olfactory tract (see page 783). The olfactory 

 tract, when traced backward, divides into three slips or roots at its base. The 

 middle root is attached to the under surface of the frontal lobe, just in front of 

 the anterior perforated space. The external root passes outward, round the 

 anterior perforated space, across the fissure of Sylvius to the temporo-sphenoidal 

 lobe. The internal root passes inward, and joins the lower end of the gyrus 

 fornicatus. 



These three roots unite and form a flat band, narrower in the middle than 

 at either extremity, and of a somewhat prismoid form on section. It is soft in 

 texture and contains a considerable amount of gray matter in its substance. As it 

 passes forward it is contained in a deep sulcus, the olfactory suleus, between two 

 convolutions, lying on the under surface of the frontal lobe, on either side of the 

 longitudinal fissure, and is retained in position by the arachnoid membrane, which 

 covers it. On reaching the cribriform plate of the ethmoid bone it expands into 

 the olfactory bulb. From the under part of this bulb are given off the olfactory 

 nerves, which pass through the cribriform foramina and are distributed to the 

 mucous membrane of the nose. Each nerve is surrounded by a tubular prolonga- 

 tion from the dura mater and pia mater ; the former being lost on the periosteum 

 lining the nose ; the latter, in the neurilemma of the nerve. The nerves, as they 

 enter the nares, are divisible into three groups : an inner group, larger than those 

 on the outer Avail, spread out over the upper third of the septum ; a middle set, 

 confined to the roof of the nose ; and an outer set, which are distributed over the 

 superior and middle turbinated bones and the surface of the ethmoid in front 

 of them. As the filaments descend they unite in a plexiform network, and are 

 believed by most observers to terminate in the cells of Schultze. 



The olfactory differ in structure from other nerves in being composed exclu- 

 sively of non-medullated fibres. They are deficient in the white substance of 

 Schwann, and consist of axis-cylinders, with a distinct nucleated sheath, in which 

 there are, however, fewer nuclei than in ordinary non-medullated fibres. 



