NERVOUS TISSUE 141 



included in Fig. 130, in which parts derived from dorsal roots are unshaded ; 

 those from lateral roots are black; and those from ventral roots are cross- 

 hatched. They may be briefly described as follows. 



The olfactory nerve, on either side of the head, consists of about twenty separate 

 bundles of processes from the neuro-epithelial cells in the nasal mucous membrane. 

 These bundles of neuro-epithelial fibers pass directly into the olfactory bulbs, which are 

 portions of the brain. The wmero-nasal nerve is a bundle much longer than the others, 

 which arises from a tubular epithelial pocket in the mucous membrane of the nasal sep- 

 tum. This pocket is a rudimentary organ of considerable interest, known as the vomero- 

 nasal (or Jacobson's) organ. Associated with the vomero-nasal nerve, but said to be 

 distinct from it, there is a small ganglionated nerve which sends its fibers into the 

 brain caudal to the olfactory lobe. Distally it is "distributed chiefly to the vomero- 

 nasal organ." This is the Nervus terminalis, discovered in fishes by Pinkus in 1894, 

 and recently found in human and pig embryos and in adult dogs and cats (Johnston, 

 Journ. Comp. Neur., 1913, vol. 23, pp. 97-120; and McCotter, ibid., pp. 145-152). 



The optic nerve is a round cord of fibers extending from ganglion cells in the retina 

 to the brain. It is quite unlike any portion of a spinal nerve, and will be described 

 in connection with the eye. 



The oculomotor nerve has only a ventral root, and consequently it is entirely motor. 

 It is distributed to four of the six muscles which move the eye-ball (namely, the inferior 

 oblique and the superior, medial and inferior rectus muscles) and to the muscle which 

 raises the upper eye-lid (M. levator palpebra superioris). 



The trochlear nerve arises from cells in the ventral part of the medullary tube, but 

 its fibers, instead of passing directly outward, grow to the dorsal surface of the tube and 

 cross to the opposite side before they emerge. Although the trochlear nerve must 

 be regarded as a ventral root, its fibers leave the brain more dorsally than those of 

 any other nerve. They come out at the notch or isthmus between the mid-brain and 

 the hind-brain, and all of them pass to the superior oblique muscle of the eye-ball. 

 This muscle, which runs through a fibrous ring or pulley (trochlea) attached to the 

 frontal bone, turns the eye outward and downward. 



The trigeminal nerve consists of dorsal and lateral roots. Its sensory cells form the 

 semilunar ganglion, which gives rise to three large nerves, the ophthalmic, maxillary 

 and mandibular (hence the name trigeminal}. In general terms, the ophthalmic is 

 the sensory nerve of the forehead and largely of the scalp; the maxillary is the sensory 

 nerve of the front of the face and the upper teeth; and the mandibular distributes 

 sensory fibers to the front of the tongue, the lower teeth, and the skin over the lower 

 jaw. Unlike the ophthalmic and maxillary nerves, the mandibular is a mixed nerve, 

 receiving all the motor fibers of the trigeminal. These motor fibers are distributed" 

 chiefly to the muscles of mastication, through the masticator nerve. 



The abducent nerve is wholly a ventral root, and its fibers all pass to the lateral rec- 

 tus muscle, which abducts the eye-ball (i.e., turns it outward). 



The facial nerve is largely a lateral root, and is the motor nerve of the facial muscles. 

 It has, however, a dorsal root (the so-called Nervus intermedius) and a ganglion 

 known as the ganglion geniculi, or geniculate ganglion, since it occurs at a bend in 

 the nerve. The facial nerve has three fundamental branches, all of which contain 

 both sensory and motor fibers; these are the large superficial petrosal nerve, the chorda 

 tympani, and the facial nerve (the name of the entire nerve being applied to one of 

 its parts). 



The acoustic nerve, which is wholly associated with the internal ear, is entirely sen- 



