720 THE NERVOUS SYSTEM. 



the cerebellum. This tract seems to lose some of its fibres in the cord, since the 

 area of its degeneration resulting from a section of the lower part of the cord 

 diminishes from below upward ; only some of its fibres therefore pass directly to 

 the cerebellum. On the other hand, the tract is reinforced by an accession of fibres 

 from the cord itself, so that its transverse area is greater above than below. 



The antero-lateral ascending tract of Gower arises in the cord, probably as the 

 axis-cylinders of cells situated in the posterior horn. Passing across the middle 

 line through the anterior gray commissure the fibres ascend in the antero-lateral 

 column of the cord, and ultimately reach the cerebellum through its superior 

 peduncles. 1 



THE CRANIAL NERVES. 



The cranial nerves arise from some part of the cerebrospmal 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. Trochlear (Pathetic). 10th. Pneumogastric (or Vagus). 



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. 



The First Nerve (Fig. 350, page 654). 



The First cranial or the Olfactory nerves (nn. ol/actorii), the special nerves of 

 the sense of smell, are about twenty in number. They are given oft' 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. The olfactory 

 tract and bulb have already been described (page 654). 



Each nerve is surrounded by a tubular prolongation 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 two groups : an inner group, larger than those on the outer wall, spread out 

 over the upper third of the septum ; and an outer set, which is distributed over 

 the superior turbinated bone, and the surface of the ethmoid in front of it. As 

 the filaments descend, they unite in a plexiform network, and are believed by most 

 observers to terminate by becoming continuous with the deep extremities of the 

 olfactory cells. 



The olfactory differs in structure from other nerves in being composed 

 exclusively 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. 

 The olfactory centre in the cortex is not definitely known. It is generally asso- 



1 Testnt describes the ascending column of Gower as joining with the fillet, and through it being 

 carried to the cerebral cortex. 



