FIG. 8.-View of the Cerebro-spinal Axis. (After Bourgery.) 15. The right half of the cranium and trunk has been removed, and the roots of the spinal nerves dissected out and laid on their several vertebræ. F, T, O, cerebrum; C, cerebellum; P, pons Varolii; mo, medulla oblongata; ms, ms, upper and lower extremities of the spinal marrow. CVIII. are cervical nerves; DI. to DXII., dorsal; LI. to LV., lumbar; SI. to SV., sacral; Col., coccygeal.

CI. to

group belong the nerves that supply the principal muscles of the eyeball, the muscles of facial expression and of the tongue, and the so-called spinal accessory nerve. To the third group belong the three nerves which are distributed so widely over the mucous membranes and muscles of the face, tongue, pharynx, and internal organs, namely, the trigeminus, the glossopharyngeal, and the pneumogastric or vagus (see Figs. 15 and 18).

By these pairs of peripheral nerves the sensory areas of the body have the impulses originating in them conducted to the central nervous mass, while the motor areas are controlled from the same central mass. Thus sensation and perception, which are dependent upon the excited condition of the end-organs of sense, are made possible. And by reflex and automatic action of the cerebro-spinal axis both involuntary and voluntary movements of the trunk and limbs are accomplished through the conducting nerves.

STRUCTURE OF THE SPINAL CORD.

The Spinal Cord extends in the spinal canal from the aperture (foramen magnum), through which it connects with the brain, downward to opposite the body of the first lumbar vertebra. Here it tapers off into a slender thread of gray nervous substance (filum terminale). Its length is, in the adult, from fifteen to eighteen inches. It is nearly cylindrical in shape, its front and back surfaces being somewhat flattened. It has two considerable enlargements of its girth, an upper (cervical), from which arise the nerves that supply the upper limbs, and a lower (lumbar) which supplies the lower limbs with nerves.

Fissures and Commissures of the Cord. The spinal cord is almost completely divided for its entire length into right and left halves by two "median" Fissures; a somewhat broader one in front (anterior), and a somewhat deeper but narrower one behind (posterior). Two bands

FIG. 9. A, Anterior, and B, Posterior, View of the Spinal Cord and Medulla Oblongata. B', the Filum terminale, which has been cut off from A and B. 1, Pyramids of the medulla, and 1', their decussation. 2, olives; 3, lateral strands of the medulla; 4', calamus scriptorius; 5, the funiculus gracilis; and 6, the funiculus cuneatus; 7, the anterior, and 9, the posterior, fissures; 8, the antero-lateral depression; 10, postero-lateral groove. C, the cervical, and L, the lumbar, enlargements of the cord."

of nervous matter, called Commissures, unite its halves and prevent the fissures from dividing it completely. The front one, at the bottom of the anterior fissure, is the white commissure; the one behind, at the bottom of the posterior fissure, is the gray commissure. The latter is larger than the former, except at the cervical and lumbar enlargements of the cord. Along its entire length the gray commissure encloses a minute canal (central canal).

Columns and Horns of the Cord. Each half of the cord is subdivided by the entrance of the nerve-roots into three

FIG. 10.-A, Anterior, and B, Lateral, View of a Portion of the Cord from the Cervical Region. 2/1 (Schwalbe.) 1, Anterior median, and 2, posterior median, fissures. At 3, is the antero-lateral depression, over which spread the anterior roots (5). The posterior roots (6), with their ganglion (6'), arise from the postero-lateral groove, and uniting with the anterior roots form the compound nerve (7).

Columns. These

are (a) the anterior, which lies between the anterior median fissure and the anterior roots; (b) the posterior, which lies between the posterior median fissure and the posterior roots; and (c) the lateral, which lies at the side of the cord between the other two columns.

Transverse sections of the cord show that its substance, like all

nervous substance, consists of both white and gray nervous matter. The former is external and composes the columns of the cord; the latter is internal and is surrounded by the white matter. The relative amount of the

two varies in different portions of the cord. At the lower end of the cord scarcely any white matter is found; the amount of such matter increases from below upwards, and is largest in the cervical region. The amount of gray matter is greatest in the two enlargements of the cord.

The gray columns and their commissures form a figure somewhat like a Roman H or a large X, or a pair of butterfly's wings. But the lateral masses of these crescentshaped bodies are narrower in the thoracic region, and broader at the cervical and lumbar enlargements. The limbs of the figure thus formed are called Horns; of these horns each side has therefore two, a rounded anterior and a longer and narrower posterior horn. The division into columns, fairly well marked on the surface, is lost as we pass into the central gray matter. The anterior horn has here an appearance of "spongy substance"; the posterior, of a kernel of such substance surrounded by “gelatinous substance."

White Substance of the Cord. The external or white matter of the spinal cord, besides connective tissue and lymph- and blood-vessels, is composed of nerve-fibres. The essential part of these fibres is the axis-cylinder; although, when fully developed, they have also the medullary sheath. They vary in size, the thickest being found in the outer portions of the anterior columns (1200 to 200 of an inch). In the lateral columns the finer ones lie near the gray matter; but in the posterior columns they increase in thickness as they approach the posterior gray commissure.

The direction of the nerve-fibres in the white substance of the cord is either vertical, or horizontal, or oblique. The vertical fibres are most abundant and are united into fascicles which ascend toward the brain. The horizontal fibres are either commissural or fibres of the roots. The fibres of the white commissure run horizontally along the median border of the gray matter of the horns, and become