. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 48 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tral rami interlace to form a plexus (cervical and brachial in front, lumbar and sacral for the hind limb), from which nerves are distributed to the appendage.^ The spinal cord is enlarged where the spinal ner\'es forming these plexuses are given off. In the early stages the spinal cord is as long as the region of the body sup- plied by it, but with increase in size the other tissues grow faster than the cord. As a result, the more posterior spinal nen-es take a very oblique course
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 48 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. tral rami interlace to form a plexus (cervical and brachial in front, lumbar and sacral for the hind limb), from which nerves are distributed to the appendage.^ The spinal cord is enlarged where the spinal ner\'es forming these plexuses are given off. In the early stages the spinal cord is as long as the region of the body sup- plied by it, but with increase in size the other tissues grow faster than the cord. As a result, the more posterior spinal nen-es take a very oblique course, while the hinder end of the cord is drawn out into a very slender thread, the filum terminale. The large bundle of nerves hich consequently extends be- hind the cord forms what is known as a cauda equina. The brain is an enlarged and immensely complicated ante- ^^St^.^ ^^ Vv^^^^^K"^^ central nervous system, and }'et we can trace in it some of the constituents we have come to rec- ognize in the spinal cord. \'ery soon after the clo- sure of the neural tube the region which is to form the brain becomes differentiated into three hollow enlargements or vesicles, which have received the names, according to position, of fore, mid, 1 .\ brachial plexus is formed in snakes and footless lizards. None is found in the c£cilians. Siren lacks a sacral Fig. 49. Right human cervical and brachial plex- uses (from Martin) showing the interlacing of nerve trunks. C, I- VII, roots of cervical nerves; D, I-III, three anterior dorsal roots; 1-4, nerves of cervical plexus; 4', phrenic nerve (to diaphragm) ; <, circumflex; r, musculo-cutaneus; ic, internal cutaneus; /«, median; i, intercostals; «, 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 Kingsley, J. S. (John Sterling), 1854-1929. New York, H.
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