ference, and for their transmission to the ganglionic centres; whilst the other serves for the transmission of the impressions that call forth Muscular contraction, from the ganglionic centres to the various parts of the circumference.-In most Nerve-trunks, afferent and motor fibres are bound up together; although, in the ordinary Spinal nerves of Vertebrata, these are connected by separate "roots" with the Spinal Cord which serves as their ganglionic centre (§ 62). But the nerves of special sense (the Olfactive, Optic, and Auditory), which proceed to those special ganglionic centres of which the aggregate constitutes the Sensorium, contain no motor fibres; and there are other nerves of the head in Vertebrata, which are either solely afferent or solely motor (Fig. 11). 40. The analogy just indicated between the two components of every Nervous System, and the two parts of an Electric Telegraph, —that in which change originates, and that which serves as the conductor,-holds good to this further extent; that as, for the origination of the Electric current, a certain Chemical reaction must take place between the exciting liquid and the galvanic combination of metals, so is it necessary, for the production of Nerve-force, that a reaction should take place between the Blood, on the one hand, and either the central nerve-cells, or the peripheral expansions of the nerve-fibres. We do not know, it is true, what is the precise nature of that reaction: but we have the evidence of it in the large supply of Blood which goes to all Organs of Sense, ie., to organs which are adapted for receiving sensory impressions and transmitting them to the central Sensorium; and, yet more, in the extraordinary proportion that is transmitted to those central organs which receive those impressions, render the Mind cognizant of them as Sensations, and furnish the instrumental conditions of all Psychical operations, as well as of their action upon the Body. Thus, in the case of Man, although the Brain has not ordinarily more than about one-fortieth of the weight of the body, yet it is estimated to receive from one-sixth to one-fifth of the whole circulating Blood. 41. The immediate dependence of the production of Nerve-force upon a reaction between the Nerve-substance and the Blood, is proved by the effects of suspension of the circulation, whether local or general. Thus, if the supply of blood to a limb be temporarily interrupted (as by pressure on its main artery), numbness, or diminution of Sensibility, is perceived in it, as well as loss of Muscular power (the hand or foot being "asleep"), until the circulation is re-established. The effect of complete interruption to the blood-supply of the Brain is extremely remarkable. That supply is conveyed into the cavity of the skull of Man and of the higher Vertebrata by four arterial trunks, which enter it at no great distance from one another, and then unite into the "circle of Willis;" from which are given off the various branches that distribute arterial blood to every part of the brain-substance. After traversing this, the blood returns by the veins, greatly altered in its chemical composition; especially as regards the loss of free Oxygen, and its replacement by various oxy-compounds of Carbon, Hydrogen, Phosphorus, &c., that have been formed by a process analogous to combustion. Now if one, two, or three of the arterial trunks be tied, the total quantity of blood supplied to the brain is diminished; but in virtue of the "circle of Willis," no part is entirely deprived of blood; and the functional activity of the brain, though enfeebled, is still maintained. If, however, the fourth artery be compressed so as entirely to prevent the passage of blood, there is an immediate and complete suspension of activity, the animal becoming as unconscious as if it had been stunned by a severe blow; whilst it recovers as soon as the blood is again allowed to flow through the artery. In fact, the "stunned" state produced by a blow on the head, is only secondarily dependent upon the effect of that blow on the Brain, which may have sustained no perceptible injury whatever; the state of insensibility being due to the paralysis of the Heart and suspension of the Circulation, induced by the "shock." For the like insensibility may be the result of a blow on the "pit of the stomach" (acting on the great Solar plexus of nerves, § 112), or of the shock of some overpowering mental Emotion, either of which produces the like paralysis of the heart. Further, if the blood transmitted to the brain, though not deficient in quantity, be depraved in quality by the want of Oxygen and the accumulation of Carbonic acid (as happens in Asphyxia), there is a gradually increasing torpor of the mental faculties, ending in complete insensibility. (See also § 472, and Appendix.) : 42. Thus, then, the dependence of Nervous power and of Mental activity upon the Physical changes kept up by the Circulation of oxygenated Blood through the brain, can be shown experimentally to be just as direct and immediate, as is the dependence of the Electric activity of a Galvanic battery upon the analogous changes taking place between its Metals and its exciting Liquid. And if we say that Electricity is the expression of Chemical change in the one case, how can we refuse to regard Thought as the expression of Chemical change in the other ?—This view is not here advanced as explaining any Mental phenomenon. No Physicist would say that he can "explain explain" how it is that Electricity is generated by Chemical change but he knows that such a relation of cause and effect exists between the two orders of phenomena, that every Chemical change is accompanied by a disturbance of Electricity; and thus, whenever he witnesses Electric disturbance, he is led to look for some Chemical change as its Physical cause. And in precisely the same sense, and no other, the Physiologist must regard some change in the substance of the Brain as the immediate Physical antecedent of all automatic Mental action. It is the attribute of the Will to utilize this automatic power of the Brain, as it utilizes that of the Muscles; and thus to make the Ego, in proportion as he has acquired the mastery over it, a "free agent (§§ 25-28). SECTION 3. Different Forms and Modes of Action of the 43. The simplest type of an Animal consists of a minute mass of protoplasm" or living jelly, which is not yet differentiated into "organs;" every part having the same endowments, and taking an equal share in every action which the creature performs. One of these "jelly-specks," the Amaba (Fig. 4), moves itself about by changing the form of its body, extemporising a foot (or pseudopodium) first in one direction and then in another; and then, when it has met with a nutritive particle, extemporises a stomach for its reception, by wrapping its soft body around it. Another, instead of Fig. 4. AMOEBA IN DIfferent forMS, A, B, C, going about in search of food, remains in one place, but projects its protoplasmic substance into long pseudopodia (Fig. 5), which entrap and draw-in very minute particles, or absorb nutrient material from the liquid through which they extend themselves, and are continually becoming fused (as it were) into the central body, which is itself continually giving off new pseudopodia.-Now we can scarcely conceive that a creature of such simplicity should possess any distinct consciousness of its needs, or that its actions should be directed by any intention of its own; and yet the Writer has lately found results of the most singular elaborateness to be wrought-out by the instrumentality of these minute "jelly-specks,". Fig. 5. GROMIA, WITH EXTENDED PSEUDOPODIA. which build-up "tests" or casings of the most regular geometrical symmetry of form, and of the most artificial construction. a. Suppose a Human mason to be put down by the side of a pile of stones of various shapes and sizes, and to be told to build a |