--

THE SPINAL NERVES.

19

the corresponding segments of the cord-that is, from the eight cervical, twelve thoracic, five lumbar segments, etc. Each nerve arises by two roots, which leave the spinal cord separately. The anterior motor roots emerge between the anterior and lateral columns, while the posterior sensory roots leave the cord between the lateral column and the posterior column. Before the two roots become united in the nerve the posterior root forms a node-like enlargement the intervertebral ganglion which is lodged in the intervertebral foramen. The nerves for the neck and extremities form numerous anastomoses with each other, and thus produce the nervous plexuses (cervical, brachial, lumbar, sacral, etc.) before they are distributed to the soft parts. Strictly speaking, the peripheral nerves originate in these plexuses. From this arrangement it follows that the peripheral nerves contain mixed fibers from the anterior and posterior roots of several spinal segments. (See Plate 27.) The nerve-roots for the lower extremity, owing to their high origin at the level of the first lumbar vertebra (see Plate 27), continue their downward course through the intervertebral foramina for a certain distance before they make their exit, at first accompanying the conus medullaris, and lower down forming the cauda equina.

The gangliated cord of the sympathetic consists of a series of small ganglia arranged along the anterior surface of the vertebral column. The highest of these are the superior, middle, and inferior cervical ganglia, the ganglia being connected with one another by slender bundles of nerve-fibers. The ganglia are joined by numerous nervefibers derived from the cranial nerves and from the spinal plexuses. The main trunk of the sympathetic terminates on the coccyx in the ganglion impar. The sympathetic nerves, after emerging from the main trunk and its ganglia, either follow the blood-vessels or join the cranial and spinal nerves and are distributed to the organs which they supply-namely, all smooth muscle-fibers. Many of them

first unite to form plexuses in the neighborhood of the viscera (cardiac, mesenteric, intestinal ganglia, etc.).

The arteries that supply the brain are derived from the internal carotid and vertebral arteries and the arterial circle of Willis, which is formed by their anastomoses. (See Plate 5.) The branches of the circle of Willis, which occupy the pia, are the following: Art. corp. callosi (corpus callosum, median surfaces of the hemispheres); art. fossæ Sylvii (tissues surrounding the fossa and basilar

Figures 4 and 5 show the blood supply; figure 5 that of the art. fossæ Sylvii (A.f.S.), which supplies the basal ganglia and the inner capsule (a.l.st, art. lenticulostriata, a very important branch in pathologic lesions). Figure 4 represents a transverse section of the spinal cord with its blood-vessels. Note the manner in which the gray substance is supplied by the art. sulci anterior.

ganglia); art. choroidea (ventricles); art. profunda (occipital and temporal lobes), etc. The blood-vessels that supply the central white matter of the brain are so-called end vessels-that is to say, their anastomosis is not sufficient to permit neighboring vessels to establish collateral circulation. The cortical arteries, however, which enter from the pia anastomose freely with one another.

the

The arteries of the spinal cord are derived partly from arteries-art. spinal., ant. and post-and

BLOOD-SUPPLY OF BRAIN AND SPINAL CORD. 21

partly from the intercostal arteries. They enter the cord from all directions, but especially through the anterior longitudinal sulcus, and are also end vessels-that is, they end blindly.

The returning blood from the brain is carried by the pial veins into the venous sinuses, which empty into the internal jugular vein. The blood from the third ventricle is collected by the veins of Galen.

The veins of the spinal cord anastomose freely with one another and form Breschet's venous plexus, which surrounds the dura mater.

The lymphatic channels of the brain and spinal cord (the external coat of the vessel forms the "adventitious lymphatic sheath") communicate with the lymph-spaces formed by the meninges. The ventricles, therefore, are in direct communication with the subarachnoid space and contain the cerebrospinal fluid.

The nerves of the pia and dura are derived from the sympathetic system, with the exception of some in the dura which are branches of the sensory portion of the trifacial nerve.

PART II.

DEVELOPMENT AND STRUCTURE OF THE NERVOUS SYSTEM.

(Plates 15 to 53.)

THE central nervous system is derived from the ectoderm. In the center of the germinal area two (“ medullary") folds of ectodermic tissue make their appearance, and as they approach each other dip down to form the medullary groove. This groove is later converted into the medullary canal by the union of the medullary folds along the dorsal line. The neuroglia, or sustentacular tissue of the nervous system, and the essential nerve substance are developed from the epithelial layers which form the lining of this neural canal. The meninges and bloodvessels are derived from the mesoderm.

Laterally and dorsally from the medullary groove a row of cells (the ganglionic cord) makes its appearance and later assumes a position by the side of the medullary canal along its entire extent. These masses of cells are the ancestors of the intervertebral ganglia and the homologous ganglia of the sensory cranial nerves (olfactory, Gasserian, cochlear, jugular, petrosal ganglia, etc.).

The nasal, club-like extremity of the medullary canal during the first embryonal month expands into three primary brain vesicles, the first and the third of which soon subdivide into two secondary compartments: The primary forebrain (subdividing into the forebrain

PRIMARY SUBDIVISION OF THE BRAIN.

23

and interbrain), the midbrain, and the hindbrain (the latter subdividing into the hindbrain and the afterbrain); from these three, or more correctly five, primary vesicles the entire brain is later developed. The cavity of the medullary canal becomes converted into the ventricular system, and the thickened walls of the canal and of the vesicles eventually form the structures of the spinal cord and brain.

By the ingrowth of the falciform process of the dura, invaginating the roof of the brain vesicle (pallium), the forebrain is divided into two lateral halves (primitive cerebral hemispheres), which, by virtue of their more rapid growth, soon overlap all the other portions of the brain. The thickening of the walls of the forebrain vesicle results in the formation of the cortex and cerebral medulla, while its primary cavity is converted into the lateral ventricle, into which the primitive corpus striatum projects. The latter eventually unites with the lateral wall at the external capsule, while the median surface remains free (caudate nucleus).

The derivatives of the lateral wall of the forebrain are the cortex and white matter of the convex surfaces, the outer capsule, the caudate nucleus, and the putamen of the lenticular nucleus. The derivatives of the mesial wall are the globus pallidus of the lenticular nucleus, the lamina. cornea, fornix, septum lucidum, and nervi Lancisi. The internal capsule is formed later by union with the optic thalamus at the neural end plate.

At the median surface of the vesicles which form the hemispheres the corpus callosum makes its appearance, and, bridging over the median line, unites with the corresponding structure of the opposite side.

The earliest fissures and convolutions are formed between the second and third months by invaginations of the cortex, the fissure of Sylvius being the first to make its appearance. (See Plate 16.)

From the interbrain are derived the optic thalami and