. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 533 brachial, lumbar, and sacral. Frequently the four unite in pairs to form cervico-thoracic and lumbo-sacral plexuses. In cyclostomes and fishes the group of postoccipital nerves forms a plexus which innervates the hypobranchial muscles. In many forms this cervical plexus unites with the brachial plexus. In amniotes the two plexuses become separate and the plexus of hypobranchial nerves forms the hypoglossus (XII) Fig. 440.—A, diagram of collector nerve; B, of a nerve plexus. (After Braus.) C, brachial plexus of Salamand
. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 533 brachial, lumbar, and sacral. Frequently the four unite in pairs to form cervico-thoracic and lumbo-sacral plexuses. In cyclostomes and fishes the group of postoccipital nerves forms a plexus which innervates the hypobranchial muscles. In many forms this cervical plexus unites with the brachial plexus. In amniotes the two plexuses become separate and the plexus of hypobranchial nerves forms the hypoglossus (XII) Fig. 440.—A, 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 Ftirbringer.) In different species, and even in different individuals of the same species, there is considerable variation in the number of nerves in a plexus and in their interconnexions, so that exact homologies are impossible. The largest number of nerves in the cervico-thoracic plexus, twenty-five, occurs in skates. In higher vertebrates the number is usually reduced to four or five, and in man, the cervical plexus includes only the first four spinal nerves. The brachial plexus in man usually involves the lower four cervical and the first thoracic nerves. The first four lumbar nerves of man unite in the lumbar plexus; the fifth lumbar and the five sacral nerves form the sacral plexus. But generalization is difficult because of the large amount of variation. Such plexuses are presumably adaptive, since by the interlacing of fibers any muscle may be innervated by more than a single nerve and a summation of stimulation effected. This does not imply, however, that. 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. B
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