though not of large size, has very important functions and connections. This is known as the ganglion of the tuber annulare.

Situated almost immediately above these parts we have the corpora striata in front, and the optic thalami behind, nearly equal in size, and giving passage, as above described, to the fibres of the anterior and posterior columns. Behind them still, and on a little lower level, are the tubercula quadrigemina, giving origin to the optic nerves. The olfactory ganglia rest upon the cribriform plate of the ethmoid bone, and send the olfactory filaments through the perforations in this plate, to be distributed upon the mucous membrane of the upper and middle turbinated bones. The cerebellum covers in the fourth ventricle and the posterior surface of the medulla oblongata; and finally the cerebrum, which has attained the size of the largest ganglion in the cranial cavity, extends so far in all directions, forward, backward, and laterally, as to form a convoluted arch or vault, completely covering all the remaining parts of the encephalon.

Fig. 131.

The entire brain may therefore be regarded as a connected series of ganglia, the arrangement of which is shown in the accompanying diagram. (Fig. 131.) These ganglia occur in the following order, counting from before backward: 1st. The olfactory ganglia. 2d. The cerebrum or hemispheres. 3d. The corpora striata. 4th. The optic thalami. 5th. The tubercula quadrigemina. 6th. The cerebellum. 7th. The ganglion of the tuber annulare. And 8th. The ganglion of the medulla oblongata. Of these ganglia, only the hemispheres and cerebellum are convoluted, while the remainder are smooth and rounded or somewhat irregular in shape. The course of the fibres coming from the anterior and posterior columns of the cord is also to be seen in the accompany. ing figure. A portion of the anterior fibres, we have already observed, pass upward and backward, with the restiform bodies, to the cerebellum; while the remainder run forward through the tuber

Diagram of HUMAN BRAIN, in vertical section; showing the situation of the different gan

glia, and the course of the fibres. 1 Olfactory

ganglion. 2. Hemisphere. 3. Corpus striatum.

4. Optic thalamus. 5. Tubercula quadrigemina.

6. Cerebellum. 7. Ganglion of tuber aunulare. 8. Ganglion of medulla oblongata.

annulare and the corpus striatum, and then radiate to the gray matter of the cerebrum. The posterior fibres, constituting the restiform body, are distributed partly to the cerebellum, and then pass forward, as previously described, underneath the tubercula quadrigemina to the optic thalami, whence they are also finally distributed to the gray matter of the cerebrum.

The cerebrum and cerebellum, each of which is divided into two lateral halves or "lobes," by the great longitudinal fissure, are both provided with transverse commissures, by which a connection is established between their right and left sides. The great transverse commissure of the cerebrum is that layer of white substance which is situated at the bottom of the longitudinal fissure, and which is generally known by the name of the "corpus callosum." It consists of nervous filaments, which originate from the gray matter of one hemisphere, converge to the centre where they become parallel, cross the median line, and are finally distributed to the corresponding parts of the hemisphere upon the opposite side. The transverse commissure of the cerebellum is the pons Varolii. Its fibres converge from the gray matter of the cerebellum on one side, and pass across to the opposite; encircling the tuber annulare with a band of parallel curved fibres, to which the name of "pons Varolii" has been given from their resemblance to an arched bridge.

The cerebro-spinal system, therefore, consists of a series of ganglia situated in the cranio-spinal cavities, connected with each other by transverse and longitudinal commissures, and sending out nerves to the corresponding parts of the body. The spinal cord supplies the integument and muscles of the neck, trunk, and extremities; while the ganglia of the brain, beside supplying the corresponding parts of the head, preside also over the organs of special sense, and perform various other functions of a purely nervous character.

CHAPTER II.

OF NERVOUS IRRITABILITY AND ITS MODE OF

ACTION.

WE have already mentioned, in a previous chapter, that every organ in the body is endowed with the property of irritability; that is, the property of reacting in some peculiar manner when subjected to the action of a direct stimulus. Thus the irritability of a gland shows itself by increased secretion, that of the capillary vessels by congestion, that of the muscles by contraction. Now the irritability of the muscles, indicated as above by their contraction, is extremely serviceable as a means of studying and exhibiting nervous phenomena. We shall therefore commence this chapter by a study of some of the more important facts relating to muscular irritability.

The irritability of the muscles is a property inherent in the muscular fibre itself. The existence of muscular irritability cannot be explained on any known physical or chemical laws, so far as they relate to inorganic substances. It must be regarded simply as a peculiar property, directly dependent on the structure and constitution of the muscular fibre; just as the property of emitting light belongs to phosphorus, or that of combining with metals to oxygen. This property may be called into action by various kinds of stimu lus; as by pinching the muscular fibre, or pricking it with the point of a needle, the application of an acid or alkaline solution, or the discharge of a galvanic battery. All these irritating applications are immediately followed by contraction of the muscular fibre. This contraction will even take place under the microscope, when the fibre is entirely isolated, and removed from contact with any other tissue; showing that the properties of contraction and irritability reside in the fibre itself, and are not communicated to it by other parts.

Muscular irritability continues for a certain time after death. The stoppage of respiration and circulation does not at once destroy the vital properties of the tissues, but nearly all of them retain these properties to a certain extent for some time afterward. It is only when the constitution of the tissues has become altered by

being deprived of blood, and by the consequent derangement of the nutritive process, that their characteristic properties are finally lost. Thus, in the muscles, irritability and contractility may be easily shown to exist for a short time after death by applying to the exposed muscular fibre the same kind of stimulus that we have already found to affect it during life. It is easy to see, in the muscles of the ox, after the animal has been killed, flayed, and eviscerated, different bundles of muscular fibres contracting irregularly for a long time, where they are exposed to the contact of the air. Even in the human subject the same phenomenon may be seen in cases of amputation; the exposed muscles of the amputated limb frequently twitching and quivering for many minutes after their separation from the body.

The duration of muscular irritability, after death, varies considerably in different classes of animals. It disappears most rapidly in those whose circulation and respiration are naturally the most active; while it continues for a longer time in those whose circulation and respiration are sluggish. Thus the muscular irritability in birds continues only a few minutes after the death of the animal. That of quadrupeds lasts somewhat longer; while in reptiles it remains, under favorable circumstances, for many hours. The cause of this difference is probably that in birds and quadrupeds, the tissues being very vascular, and the molecular changes of nu trition going on with rapidity, the constitution of the muscular fibre becomes so rapidly altered after the circulation has ceased, that its irritability soon disappears. In reptiles, on the other hand, the tissues are less vascular than in birds and quadrupeds, and all the nutritive changes go on more slowly. Respiration and circulation can therefore be dispensed with for a longer period, before the constitution of the tissues becomes so much altered as to destroy altogether their vital properties.

Owing to this peculiarity of the cold-blooded animals, their tissues may be used with great advantage for purposes of experiment. If a frog's leg, for example, be separated from the body of the animal (Fig. 132), the skin removed, and the poles of a galvanic apparatus applied to the surface of the muscle (a, b), a contraction takes place every time the circuit is completed and a discharge

Fig. 132.

FROG'S LEG, with poles of galvanic battery applied to the muscles at a, b.

passed through the tissues of the limb. The leg of the frog, prepared in this way, may be employed for a long time for the purpose of exhibiting the effect of various kinds of stimulus upon the muscles. All the mechanical and chemical irritants which we have mentioned, pricking, pinching, cauterizing, galvanism, &c., act with more or less energy and promptitude, though the most efficient of all is the electric discharge.

Continued irritation exhausts the irritability of the muscles. It is found that the irritability of the muscles wears out after death more rapidly if they be artificially excited, than if they be allowed to remain at rest. During life, the only habitual excitement of muscular contraction is the peculiar stimulus conveyed by the nerves. After death this stimulus may be replaced or imitated, to a certain extent, by other irritants; but their application gradually exhausts the contractility of the muscle and hastens its final disappearance. Under ordinary circumstances, the post-mortem irritability of the muscle remains until the commencement of cadaveric rigidity. When this has become fairly established, the muscles will no longer contract under the application of an artificial stimulus.

Certain poisonous substances have the power of destroying the irritability of the muscles by a direct action upon their tissue. Sulpho-cyanide of potassium, for example, introduced into the circulation in sufficient quantity to cause death, destroys entirely the muscular irritability, so that no contraction can afterward be produced by the application of an external stimulant.

Nervous Irritability.-The irritability of the nerves is the property by which they may be excited by an external stimulus, so as to be called into activity and excite in their turn other organs to which their filaments are distributed. When a nerve is irritated, therefore, its power of reaction, or its irritability, can only be estimated by the degree of excitement produced in the organ to which the nerve is distributed. A nerve running from the integument to the brain produces, when irritated, a painful sensation; one distributed to a glandular organ produces increased secretion; one distributed to a muscle produces contraction. Of all these effects, muscular contraction is found to be the best test and measure of nervous irritability, for purposes of experiment. Sensation cannot of course be relied on for this purpose, since both consciousness and volition are abolished at the time of death. The activity of the glandular organs, owing to the stoppage of the circulation, disappears also very rapidly, or at least cannot readily be demonstrated. The