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The Spinal Cord

lumbar, vertebra, anterior, nerves, fibers and medulla

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THE SPINAL CORD The spinal cord (medulla spinalis) is developed from the pos terior part of the neural tube, and forms the corresponding portion of the central axis of the nervous system.

Extent.—It is continuous with the medulla oblongata, above; and, in the adult, reaches to the lower border of the first lumbar vertebra (Fig. 138). Its length is 43-45 cm. (17-18 in.). In a very slender process the filum terminale internum, the cord is continued beyond the first lumbar vertebra. That process and the lower spinal nerves form the cauda equines which is inclosed in a sheath composed of the arachnoid and dura mater. The filum terminale internum for about 7.5 cm. contains a prolongation of the central gray matter and ventricle of the cord and also a few fibers which suggest the coccygeal nerves of lower animals.

In the foetus before the third month, the cord and spinal canal are of equal length. It touches the base of the sacrum at the sixth month in utero. At birth the cord reaches the third lumbar vertebra and it continues to recede with the rapid growth of the vertebra to its adult position.

Diameters (Fig. i39).—The spinal cord is shaped like a cylinder, slightly flattened from before backward (dorso-ven trally). Its longest diameter is transverse and measures less than 12 mm. (0.5 in.), except in the cervical and lumbar en largements of the cord. In the latter it equals 12-13 mm. and_in the former 15 mm. (o.6 in.). The thoracic portion of the cord is small and nearly cylindrical in shape. Divested of its meninges and nerves the spinal cord weighs about 28 grams or one ounce avoirdupois.

Though the post-natal growth of the spinal cord lags behind that of the vertebral column, its growth is relatively greater after birth than that of the brain: the brain a little more than triples its weight at birth; by extrauterine growth, the weight of the cord increases seven fold.

The cervical enlargement (intumescentia cervicalis) extends from the medulla oblongata to the second thoracic vertebra (Figs. 138 and 139). Its greatest diameter is on a level with the

fifth intervertebral disc. It gives origin to the motor fibers and receives the sensory fibers of the nerves which form the cervical and brachial plexuses.

The lumbar enlargement (intumescentia lumbalis) begins at the tenth thoracic vertebra and increases to the twelfth (Figs. 138 and 139). Opposite the first lumbar vertebra it tapers off almost to a point, the conus medullaris, but a very small process continues in the filum terminale internum. From the lumbar enlargement rise the motor fibers of the nerves contained in the lumbar and sacral plexuses, and into it enter the sensory fibers of the same plexuses.

Ventricle (Fig. 139, B).—The central canal of the spinal cord (canalis centralis spinalis) is the representative of the cavity of the neural tube. It is just visible to the naked eye, but it extends throughout the cord and expands above into the fourth ventricle. In the conus medullaris it is also dilated, forming the ventriculus terminalis (Krausei). It is lined with columnar ciliated cells which stand on a thick lamina of sub stantia gelatinosa.

Fissures of the Spinal Cord (Fig. r39).—The spinal cord is incompletely divided into symmetrical lateral halves by the anterior and the posterior median fissure.

The anterior median fissure (fissura mediana anterior) is the broader and shallower of the two (Fig. 139). It extends in length from the inferior end of the ventral surface of the pons (foramen cxcum of Vicq d'Azyr) down the anterior median line of the medulla and cord. As to depth, it equals one-third of the cord's axis. Its floor is formed by the white anterior com missure. Both layers of pia mater dip down into it and inclose the anterior spinal artery and its branches. The anterior median fissure is interrupted at the junction of the cord and medulla by the decussation of the pyramids. In the lumbar enlargement it gradually disappears.

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