. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 525 the second pair of sensory nerves of amphioxus may be the homologues of the ophthalmic nerves of craniates. The four mixed nerves posterior to the ophthalmic have been compared respectively with the trigeminal, facial, glossopharyngeal, and vagus. It is probable, however, that at least three segmental nerves are represented in the vagus nerve of verte- brates. Of the somatic motor nerves of vertebrates, the oculomotor is possibly represented in the first ventral root of CIRRI AMPHIOXUS. VELUM Fig. 436.—A diagram sh
. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 525 the second pair of sensory nerves of amphioxus may be the homologues of the ophthalmic nerves of craniates. The four mixed nerves posterior to the ophthalmic have been compared respectively with the trigeminal, facial, glossopharyngeal, and vagus. It is probable, however, that at least three segmental nerves are represented in the vagus nerve of verte- brates. Of the somatic motor nerves of vertebrates, the oculomotor is possibly represented in the first ventral root of CIRRI AMPHIOXUS. VELUM Fig. 436.—A diagram showing the distribution of the anterior nerves of amphioxus. The two anteriormost nerves of amphioxus lie anterior to the myotomes and are purely sensory. The remaining nerves are either mixed like those shown in the figure or are purely motor. The former alternate, the diagram shows, with the myotomes, while the latter are metameric in position. (Redrawn after Hatschek slightly modified.) The number of cranial nerves varies in craniotes. Fishes and amphibia have ten pairs; reptiles, birds, and mammals have twelve. These, beginning with the anteriormost, are: I. Olfactory. Special visceral afferent. II. Optic. Sensory. A specialized fiber tract of the brain. m. Oculomotor. Somatic efferent, with some visceral efferent fibers. IV. Trochlearis. Somatic efferent. V. Trigeminus. General somatic afferent and visceral efferent. VI. Abducens. Somatic efferent. VII. Facialis. Mixed visceral afferent and efferent. Special somatic afferent. Vm. Acoustic. Special somatic atTerent. IX. Glossopharyngeal. General and special visceral mixed. (?) Somatic sensory. X. Vagus. General and special visceral mixed. Special somatic afferent. XI. Accessory. (?) Somatic efferent, and general and special visceral efferent. Xn. Hypoglossal. Somatic Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appeara
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