INTERNAL CONFIGURATION.
Even with the unaided eye, one can readily distinguish gray and white substance in a transverse section of the spinal cord. When cut across, the centrally situated gray substance appears H-form in outline. The bridge of gray substance connecting the two limbs of the H, encloses the central canal, canalis centralis, which is immediately sur rounded by the substantia gelatinosa centralis and lined with ependyma. Above, the central canal widens at the transition of the spinal cord into the medulla oblongata and passes over into the fourth ventricle. Below, at the lower end of the conus terminalis, it expands into the ventriculus terminalis (Krause), becomes again narrow at the transition into the filum terminate and, finally, ends blindly.
The part of the gray bridge that passes behind the central canal is known as the commissura posterior, that which lies in front is the commissura grisea anterior. In front of the latter, between it and the bottom of the anterior • median fissure, is the commissura alba anterior.
The gray substance, in each half of the spinal cord, presents in front a thick swelling, the anterior horn or cornu anterius, and behind a more slender part, the poste rior horn or cornu posterius. Since the gray substance extends continuously throughout the entire length of the cord, the anterior and posterior horns appear in longitudinal sections as columns ; they are called also, therefore, the columnae griseae. The lateral part of the gray substance, in the lower cervical and the upper thoracic regions of the cord, becomes more independent and there forms the lateral horn or columna lateralis. In the entire cervical and upper thoracic cord, the gray substance extends into the white matter as a network of gray trabeculae and strands, which occupy the angle between the lateral and posterior horns and constitute the formatio reticularis. The posterior cornu begins ventrally as the base, then becomes narrower and forms the neck, cervix columnae posterioris ; dorsally follow the head of the horn, caput columnae, and the point, apex columnae posterioris, which latter embraces a crescentic field, the substantia gelatinosa Rolandi, and the dorsally situated marginal zone. Medial to the neck of the posterior cornu and close to the posterior commissure, one finds the nucleus dorsalis or Clarke' s column as a small group of cells within the gray substance of the upper lumbar, the entire thoracic and the lower cervical regions.
The white substance surrounds the gray and is subdivided, as already noted, into three tracts—the anterior column, between the anterior median fissure and the anterior roots, the posterior column, between the posterior median fissure and the posterior roots, and the lateral column, between the anterior and posterior roots. The posterior column
is further divided by the sulcus intermedius posterior into the medially situated fasciculus gracilis or Goll's column, and the laterally placed fasciculus cuneatus or Burdach's column.
In the essentials, the make-up of the spinal cord is the same in its various segments, the central gray substance in the characteristic H being everywhere enclosed by the white matter. The size and form of the cord in cross-sections, as well as the proportions of the gray and white substance, however, vary in the individual regions. In regard to size, the stronger development in the cervical and lumbar enlargements is at once noticeable. So far as the form is concerned, transverse sections are so charac teristic in the different regions that, within certain limits, the region from which a section has been taken can be determined from such data alone. Thus, cross-sections of the cord in the cervical region, particularly at the level of the IV—VIII nerve, and in part also in the highest thoracic segments are transversely oval ; in the thoracic region the cross-section is almost circular ; while in the lumbar region it is more quadrate, with more marked ventral flattening. The quadrate form is especially evident in the sacral and likewise in the coccygeal cord, where, however, in contrast to the lumbar region, the strongest flattening is dorsal with coincident ventral narrowing.
Regarding the proportion of the gray and the white substance, it is readily seen that the gray substance is most abundant in those segments from which the large limb nerves arise, that is in the cervical and lumbar enlargements. In these segments the great development of the anterior horns is particularly evident. The gray substance in the dorsal cord, on the contrary, is poorly developed, the H-form being here seen to best advantage. The white substance exhibits a robust development in the cervical, as well as the thoracic region. Towards the lumbar cord it progressively decreases in amount and, in the sacral region and toward the conus inedullaris, the white matter forms only a thin peripheral zone surrounding the gray matter, which at these levels con siderably exceeds in amount the white.