. Chordate anatomy. Chordata; Anatomy, Comparative. THE NERVOUS SYSTEM 377 the dorsal and the ventral nerve of each trunk segment unite to form a mixed nerve. In this way every spinal nerve possesses two "roots," a dorsal sensory and a ventral motor root. Another phylogenetic change is the loss of motor fibers in the dorsal roots. This takes place in the vertebrate series. Anamnia retain in their dorsal spinal roots the visceral motor fibers characteristic of the dorsal nerves of Amphioxus. In amniotes, however, these visceral motor fibers have their exit from the spinal cord by way
. Chordate anatomy. Chordata; Anatomy, Comparative. THE NERVOUS SYSTEM 377 the dorsal and the ventral nerve of each trunk segment unite to form a mixed nerve. In this way every spinal nerve possesses two "roots," a dorsal sensory and a ventral motor root. Another phylogenetic change is the loss of motor fibers in the dorsal roots. This takes place in the vertebrate series. Anamnia retain in their dorsal spinal roots the visceral motor fibers characteristic of the dorsal nerves of Amphioxus. In amniotes, however, these visceral motor fibers have their exit from the spinal cord by way of the ventral roots. The meaning of this shift is not clear. FiG- 334-—-4, diagram of collector nerve; B, of a nerve plexus. (After Braus.) C, brachial plexus of Salamandra maculata. (From Kingsley's "Comparative Anatomy of Vertebrates," after Fiirbringer.) Nerve plexuses are not found in Amphioxus. A cervical plexus appears in cyclostomes. In higher vertebrates four plexuses arise—cervical, brachial, lumbar and sacral. To form a plexus, the fibers of a number motor nerves unite in an interlaced network. In this way any muscle may be innervated by more than a single nerve and thus a summation of stimulation is effected. THE AUTONOMIC NERVOUS SYSTEM In the higher chordates a special system of nerves, the autonomic, is distributed to the smooth muscles of the digestive and circulatory systems. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son & Co.
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