482 



COMPARATIVE ANATOMY 



side of the cord), and they carry impulses caudad only, an indication of 

 the growing dominance of the anterior portion of the nervous system. 

 A primitive trait appears in the location of sensory ganglion cells within 

 the wall of the neural tube. 



The anterior ten pairs of nerves in Cyclostomes have their exit through 

 foramina in the cranium and hence are known as cranial nerves. It is 

 not unlikely that all correspond to anterior nerves" of amphioxus, except 

 the optic which is a fiber tract of the brain and not a true peripheral 

 nerve. The ten cranial nerves are the olfactory (I), optic (II), oculo- 

 motor (III), trochlearis (IV), trigeminal (V), abducens (VI), facialis (VII), 



Fig. 397. — Diagram of cranial nerves of lower vertebrate. Eye-muscle nerves 

 omitted; central nervous system dotted, fifth nerve represented as composed of two 

 nerves; lateralis nerves separated from the ninth and tenth nerves. I-X, cranial 

 nerves; 1-5, gill clefts; h, buccalis nerves; c, chorda tympani; g, geniculate ganglion; 

 h, hyoid nerve; i, intestinal (pneumogastric) nerve; i, jugal ganglion, I, lateral line nerve 

 of X; m, mouth; md, mandibular nerve; ml, mentalis nerve; 7nx, maxillary nerve; op, 

 ophthalmicus profundus nerve; opV, opVII, superficial ophthalmic nerves of V and 

 VII; p, palatine nerve; po, posttrematic nerves; pr, pretrematic nerves; pi, petrosal 

 ganglion; 5, semilunar (Gasserian) ganglion; sp, spiracle. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates.") 



auditory (VIII), glossopharyngeus (IX), and vagus (X). In lower 

 vertebrates the hypoglossus and spinal accessory nerves are not cranial 

 but spinal. Of the ten cranial nerves, I, II, and VIII are sensory. III, 

 IV, and VI somatic motor, and the others mixed sensory and motor. 

 The so-called nervus terminalis appears not to be an independent nerve, 

 but a component of the olfactory. The remaining neurites of the olfactory 

 are processes of neurosensory cells in the olfactory epithelium. 



The optic nerve develops in correlation with the eye, the retina, from 

 which the optic nerve fibers arise, being a segregated part of the wall 

 of the telencephalon. Sorrie of its fibers cross below the brain in front of 

 the infundibulum to form the Optic chiasma. The optic nerves, after 

 entering the wall of the diencephalon, pass by way of the optic thalami 

 to their reflex centers in the roof of the mesencephalon. 



The oculomotor, a somatic motor nerve with its nucleus or motor 

 center in the base of the midbrain, innervates four eye muscles, the 



