are its primary facts, the admitted data from which it takes its start. But they are all, as states, exceedingly complex, and involve numerous factors. Self-consciousness can no more discover all the factors which have united to form these states than simple external observation can analyze a portion of water into its constituent oxygen and hydrogen gases. Especially is it true that few of the antecedent and accompanying conditions of these complex states of consciousness can be discovered by introspection. Introspection, therefore, can never serve as the sole method for establishing a science of psychology.

Moreover, those antecedent or accompanying conditions of the states of consciousness, which physiological psychology particularly endeavors to discover, are the structure and functions of the nervous system. About these matters introspection can, as a rule, tell us nothing whatever. The physical science of physiology, with its method of external observation and experiment, must be relied upon to describe such conditions of mental phe

nomena.

It is obvious, then, how physiological psychology must combine the two methods which belong to the two sciences on which it depends. Introspective psychology must furnish us with the description of those complex states of consciousness, as such, which it is desired to explain. These furnish the problems to be solved. Physiology, on the other hand, must be relied upon for a description of the living and active nervous system, regarded as giving conditions to the origin and character of the states of consciousness. Physiological psychology, therefore, attempts to bring the two orders of phenomena, those called mental and those belonging to the nervous system, face to face. It considers them as mutually related; it endeavors, as far as possible, to unite them in terms of a uniform character, under law. Its method is to explain the phenomena of

man's sentient life as correlated with the life and growth and action, under stimuli, of his nervous system.

Divisions of the Subject. The different chapters of this book fall under three main divisions. We shall first consider the structure and functions of the nervous system from the modern mechanical point of view. In these earlier chapters we must rely upon the method of external observation and experiment as employed by the modern science of psychology. Our object will be to give a clear picture in outlines of what the nervous system of man is, and of how it acts in response to the different forms of stimuli which excite or irritate it. This work requires little reference to states of consciousness or to the nature of the mind. We shall, in the main, consider the nervous system as a purely physical mechanism. Yet even in these chapters certain important considerations bearing upon the nature of the mind and its relations to its bodily basis will indirectly come into view.

The next eleven chapters (VIII.-XVIII.) may be considered as constituting the second or main division of the. book. In these chapters the various relations which the science of physiological psychology has discovered between the states of conscious mind and the conditions of the excited nervous system, are presented in order. Such relations may conveniently be considered under three general groups or classes. The first group comprises the relations which can be established between the condition and activity of the higher nervous centres and the phenomena of conscious sensation and motion. The principal question raised under this head concerns the so-called "localization of function" in the hemispheres of the brain. The second group of relations includes the phenomena with which psycho-physics (in the more precise use of the term) attempts to deal. Such are the relations which exist between the quality, quantity, combination, and time

order of the various stimuli which irritate the nervous system, and the kind, amount, composite result, and timerelations of the mental phenomena. A third class of relations considers mind and body as dependent upon differences of age, sex, race, etc.

At the close of the more strictly scientific discussions of the book, we shall be in position to verify certain conclusions as to the nature of the human mind, and as to its general connection with the bodily organism. Some of the considerations introduced at this point will be of the kind ordinarily known as "metaphysical." We consider it scientific to postpone these questions, as well as all assumptions bearing upon them, until we have candidly and thoroughly discussed the related phenomena and the laws (or uniform ways) of their relation. But we also hold that psychology, even when it employs the physiological method, has the right, and is under obligation, to suggest and defend true conclusions as to the nature of the mind.

Benefits of the Study. It has been shown that physiological psychology can scarcely claim to be an independent science, or even a separate and definite branch of general psychology. It is, nevertheless, a most interesting, suggestive, and productive way of studying mental phenomena. For a long time the so-called "old psychology," as pursued by the introspective and metaphysical methods, made little or no advance. In a single generation, as pursued by the experimental and physiological methods, the science of psychology has been largely reconstructed.

The modern science of man emphasizes the necessity of studying his nature and development as that of a living unity. Man is known as the head of a series of physical and psychical existences. Only by considering him in this way can we have a trustworthy and adequate picture of his mental life and mental evolution. Such a consideration

the psychology which relies solely upon introspection and metaphysical speculation is unable to furnish. The actual achievements of the new science of physiological psychology — though, of course, still including many uncertainties and leaving many gaps to be filled- are a sufficient justification of its demands upon all students of the human mind. Further proof of the benefits of its study we confidently leave to the test of the student's experience.

CHAPTER I.

SUBSTANCE OF THE NERVOUS SYSTEM.

CHEMISTRY and the microscope have succeeded fairly well in analyzing the substance of the human nervous system. For this purpose it is safe, within certain limits, to direct our observation and experiment upon the lower animals, and to draw inferences from them which will apply to the case of man. The chemical constituents and minute structure of the elements which compose all nervous substance are largely the same. In describing these matters, it is not, then, so necessary to pay strict attention to the specific animal form from which the substance is derived. It is the way in which the elements are combined into organs, and the development and elaboration of function as dependent upon these organs, which constitute the marked differences between the nervous system of man and that of the lower animals.

The elements which enter into the nervous substance require to be considered in three ways: (1) as respects their chemical constitution; (2) as respects their form or structure; (3) as respects their general physiological function. For purposes of convenience and orderly arrangement we reserve the third consideration for another chapter.

CHEMISTRY OF THE NERVOUS SUBSTANCE.

There are few perfectly certain facts which can be obtained from the science of physiological chemistry, respecting the constitution of nervous matter. These facts