3. The Centres are composed of two substances: the gray and the white. The gray substance is often called the vesicular because of its abundant cells; but it has even more fibres than cells, and the white substance has also a few cells.[81] The gray substance is distributed over the surface of the brain—in the convolutions; and in various other parts of the encephalon. It surrounds the central canal which forms the ventricles of the brain and is continued as a very small cavity all down the spinal cord. Besides entering into the important and conspicuous masses known as the cerebral ganglia—(the optic thalami, and corpora striata)—the gray substance is massed in the corpora quadrigemina, crura cerebri pons varolii, and medulla oblongata. We shall have occasion to refer to each of those parts. Until modern times all the masses included in the skull under the familiar term Brain (or the technical term Encephalon) were regarded as the only centre, and also as the origin of all the nerves. Nor has this notion even yet entirely disappeared, although the spinal cord is known not to be a large nerve trunk, but a centre or connected chain of centres, structurally and functionally similar to the cranial centres. The shadow of the ancient error still obscures interpretation of the part this spinal cord plays in the sentient mechanism; and thus although the cord is universally admitted to be a centre for “sensitive impressions,” it is usually excluded from Sensation. This widespread and misleading notion will be critically examined in a future problem.
4. Beginning our survey of the cerebro-spinal axis with the Spinal Cord, we observe it to consist: 1°, of central gray substance surrounding the scarcely visible canal, which is all that remains of the primitive groove in the germinal membrane (§ [9]); 2°, irregular gray masses, called the anterior and posterior horns,[82] connected with the anterior and posterior roots of the spinal nerves; and 3°, strands of white fibres enclosing this central substance, and called the anterior lateral and posterior columns.
Like the Cerebrum, it is a double organ formed by two symmetrical halves, as the cerebrum is of two hemispheres. Each half innervates the corresponding half of the body. The cord is unlike the cerebrum in external form, though very like it in internal structure. The gray structure is mainly external in the cerebrum, and is internal in the cord.
From the anterior side of the cord (that which in animals is the under side) the motor nerves issue; from the posterior (in animals the upper) side, issue the sensory nerves. On each of the sensory nerves there is a ganglion. The roots of each nerve, formed of several rootlets issuing from the anterior and posterior columns, subsequently unite together, and proceed in a single sheath to muscles and skin, separating again, however, before they reach muscles and skin. [Fig. 2] represents this arrangement.
Fig. 2.—A portion of the spinal cord with its nerves (after Bernard). The left-hand figure shows the anterior side; the right-hand the posterior. A the anterior, and P, the posterior root, they meet at g, the ganglion; c and d are filaments connecting two posterior roots.
5. There are thirty-one pairs (sometimes thirty-two) of such nerves—namely, eight cervical, twelve thoracic, five lumbar, five sacral, and one (or two) coccygeal. [Figs. 3] to [6] represent transverse sections, which display the entrance of the roots of the nerves into the anterior and posterior horns.
6. Similar masses of gray substance in the Medulla Oblongata (which is the name given to the cord when it passes into the skull)[83] are supposed to be the origins of some other nerves (the cranial).
Fig. 3.—Transverse section of one half of the spinal cord in the lumbar region (after Kölliker). a, anterior root entering the anterior gray horns, m and l, where cells are clustered; c, central canal; d and e, the anterior and posterior commissures uniting the two halves of the cord; b, posterior root entering the posterior gray horn.