THE MTMLONAL NEBVES. 373 



or of tlie alba. Corresponding -with the origins of the large nerves forlning the brachial 

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

 enlargements which, from their location, are known as the cermal 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 medvUa 

 or metencephalon. 



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

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



§ 1007. The Myelonal or tipiiud JVerees.— These arise ixom 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 Pig. 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 ventral 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 siibooolpital and the great occipital 

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

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

 foramina they evxergs— cervical, thormic, lumbar, sacral and coccygeal or caudal (§ 463). 



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

 removed on the left to the level of the vertebral laminse. 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. rhomboideus 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. Pig. 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 tlie brain, com- 

 mencing at the foramen magnum (Pig. 55) ; see Chap. X. 



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

 with 95 per cent, alcohol (§ 283) ; 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 ; througli it pass the A. 

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



