THE MYELONAL NERVES. 373 



or of the alba. Corresponding with the origins of the large nerves forming the brachial 

 and sacral plexuses, and supplying respectively the arms and the legs, tlie myelon presents 

 enlargements which, from their location, are known as the cervical and lumbar enlarge- 

 ments. The cephalic end differs from the rest in several respects, which will be indicated in 

 Chap. X ; it is commonly regarded as a division of the brain under the name of medulla 

 or metencephaloii. 



The dorsal aspect of the cervical and of part of the thoracic myelon is represented in 

 Fig. 104 ; transections of it are given in Fig. 99, 100, 109 and 112. 



1007. The Myelonal or Spinal Nertes. These arise from the myelon or spinal cord 

 (Fig. 104) and, except in the lumbar region, pass almost directly laterad through the inter- 

 vertebral foramina, and are distributed to the tissues. If a section of the body be made at 

 the proper level, as in Fig. 109, it will be seen that the nerves are in pairs, and that each 

 nerve trunk arises by two roots the dorsal (posterior) root arising from the dorso-lateral 

 aspect of the myelon, and the ventral (anterior) root from the ventro-lateral aspect. These 

 extend toward the intervertebral foramen, piercing the dura on their way. In the foramen 

 they unite to form the nerve trunk, but just as they unite there appears on the dorsal (pos- 

 terior) root a swelling the ganglion of the dorsal root ( 999). 



The nerve trunk soon divides into two primary divisions the dorsal or posterior pri 

 mary and the 'central or anterior primary division (Fig. 109), The dorsal primary division 

 supplies the parts dorsad of the spinal column, while the ventral supplies the parts ven- 

 trad of the spinal column, including the limbs. For the most part, the ventral are much 

 larger than the corresponding dorsal divisions ; the suboccipital and the great occipital 

 nerves are, however, exceptions (see explanation of Fig. 104). The myelonal nerves are 

 sometimes designated by the names of the groups of vertebrae through whose iutervertebral 

 foramina they emerge cervical, thoracic, lumbar, sacral and coccygeal or caudal ( 483). 



Preparation Fig. 104. The muscles covering the spinal column were first carefully 

 removed on the left to the level of the vertebral laminae. The neural arch was removed 

 with nippers, entirely on the left, partly on the right. This exposed the myelun, the nerves, 

 and on the left the vertebrarterial canal with its contained vertebral artery. The left 

 nerves were then isolated by commencing at their origin and tracing the ventral primary 

 division peripherad, removing the muscles and connective tissue with the tracer, forceps 

 and scissors. The MM. rhoinboideus and serratus (magnus) were cut and the scapula 

 strongly lateriducted, to expose the brachial plexus. The parts of this plexus were then 

 carefully isolated ; (compare with the ventral view, Fig. 106). On the right, the dorsal 

 primary divisions of the nerve trunks were isolated as described for the left (see also 

 1008). They are as a rule smaller than the ventral, and hence require more care for 

 their isolation. The myelonal dura was entirely removed from the left side with the fine 

 forceps and scissors, but only to the level of emergence of the nerves on the right. 

 Finally, the skull was nipped away from the dorsal and lateral aspects of the brain, com- 

 mencing at the foramen magnum (Fig. 53) ; see Chap. X. 



In preserving this specimen, the rectum was cleared and the abdomen and thorax filled 

 with 95 per cent, alcohol ( 282) ; then it was suspended in a jar of alcohol ( 286) by a 

 cord tied to the tail and legs. Cotton was placed in the metacoelia or 4th ventricle, to 

 raise the cerebellum, and between the hemispheres, to divaricate them sufficiently to 

 show the callosum. 



Explanation of Fig. 104. Cerebellum, az. Epencephalon ; Chapter X. Costae (7) 

 The first seven ribs. Dura Dura mater of the myelon. Fm. alt., Foramen atlantale 

 The atlantal foramen in the cephalo-dorsal margin of the atlas ; through it pass the A. 

 vertebralis and the N. suboccipitalis or 1st cervical nerve. Ganglion Ganglion on the 



