THE NERVOUS SYSTEM 



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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) nerve. 



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 



