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.