heard much of "all-round" training. Verily, here is an example par excellence of an "all-round" individual. When the amoeba is affected by a stimulus-light, for example-it is not necessary that any particular portion be stimulated, for the whole body is equally sensitive. When it reacts, it does so not with a hand, a foot, a lip, a tongue, but with the entire body. It may contract one portion of its body, but it expands in another. In what direction it will move, or what part of its body will move most, is unpredictable. Just as it has no eye to be stimulated by light waves, no ear to be affected by sound waves, no special organs of touch or temperature, it does not react with a definite portion of the body and in a particular direction. A man feeling too strong a light would move his chair, pull down a curtain, turn away, or ask some one to change the conditions. That is, he would do a definite thing and bring special organs to bear in accomplishing the result. He would co-ordinate stimuli with means and modes of reacting and accomplishing definite ends. Primitive Nervous Structure. The amoeba possesses no nervous system. Zoologists have usually said that it possesses no nervous substance. But its sensitivity seems to point toward the possession of something akin to nervous material. The generalization that "there is no psychosis without neurosis" assumes that every sensitive organism must possess some nervous substance which through the action of stimuli gives rise to "neuroses," the concomitants of "psychoses." In some respects the animal possessing sensitivity is different from the plant, devoid of that quality. But the protozoans possess no system of nervous structure. Consequently, when the amoeba is affected by outside stimuli the nervous energy generated is diffused, instead of being confined to special tracts. Some of the higher protozoans, such as the slipper animalcule (paramecium) and the bell animalcule (vorticella), are somewhat more differentiated in structure and in function, but in none of the protozoans do we find anything approaching a nervous system. The Elementary Structure and Function of a Nervous System. -The purpose of the nervous system and the sense organs is

to enable the individual to gain a knowledge of the outside world through stimulation and to respond in some manner to those stimulations. The sense organs are in part, as in the retina, merely specialized portions of the nervous tissue. In part they are specialized portions of the skin so sensitized as to receive certain stimuli from the outside world. These stimuli are transformed into nervous impulses by means of the nervous system. These nervous impulses in turn become the antecedents of muscular activity and in some cases the concomitants of mental processes.

FIG. 2. A group of human nerve cells drawn to scale, X 200 diameters. A, B, C, D, F, cell bodies and the beginnings of the processes; E, cross section of a large nerve fibre. (From Donaldson's Growth of the Brain, p. 142, modified from Waller's Human Physiology.)

The fundamental elements which compose the nervous system are the neurons (Figs. 2, 3). The neurons consist of a cell body with short branching processes, the dendrites, and an axis cylinder or axon. Branching off from the axon are usually many fibrils, termed collaterals. Neurons vary greatly in size, from the minutest microscopic dimensions to three feet in length. The different neurons are not anatomically continuous, but communicate by mechanical contact only. If the neural substance of the nervous system could be entirely freed from the connective tissue and blood vessels and be much. magnified, it would present a distinctly fibrous or fibrillar appearance, rather than the jelly-like appearance so familiar in

the macroscopic view. This fibrous mass comprises many bundles of fibres and other organized pathways for the discharge of nervous energy. The muscles and their connections with the neurons complete the specialized equipment whereby we are enabled to react upon our environment.

The whole arrangement is admirably adapted for the special functions of a nervous system, viz., the liberation of nervous

B

a

FIG. 3.

A-D, showing the phylogenetic development of mature cerebral cells in a series of vertebrates: a-e, the ontogenetic development of growing cerebral cells in a typical mammal. A, frog; B, lizard; C, rat; D, man; a, neuroblast without_dendrons; b, c, developing dendrons; d and e, appearance of collaterals. (From Donaldson, op. cit., p. 146; from S. Ramén y Cajal.)

energy and the conduction of nervous impulses. To live a complex life, to be highly educated, multitudes of co-ordinations must be established between stimuli and reactions. This function the nervous system is wonderfully fitted to fulfil. Without some such mechanism, complex adjustments would be impossible.

The Reflex Arc.-The simplest sensory-neuro-muscular mechanism enabling an animal to gain definite impressions of the external world and to react in a somewhat definite manner, is

the reflex arc. This consists of (1) a specially sensitized surface or end organ, (2) a sensory neuron connected with the end organ, (3) a motor neuron, and (4) a muscle connected with the motor neuron. The accompanying diagram (Fig. 4) represents schematically the simplest reflex arc in the human spinal cord.

Beginnings of Differentiation and Organization.Among the radiata, including the echimoderms and cœlenterates, we observe much more specialization in general structure, and also the beginnings of a quite

Anterior
Root

Muscle

Motor

Nerve

Ganglion

End Organ

FIG. 4.-Schematic representation of the reflex arc.

different nervous organization. The starfish may be taken as an example (see Fig. 5). There is a ganglion at the base of each radiating arm, connected with the oesophageal ring. A branch extends from each ganglion along each arm. The star

fish possessing the beginnings of a nervous system, when stimulated, can react definitely with a particular portion of the body. Some recent experiments upon the starfish show that it can even be trained to move a particular ray upon the application of a particular stimulus. Nervous energy is directed along a particular channel and there is coordination of means and ends. This relation is only possible with a nervous system. The brain is the organ par excellence for co-ordinating functions. The medusæ possess a radiate structure similar to the starfishes, but no approach toward a central nervous organization. They possess several

FIG. 5.-Nervous system of a starfish.

r, nervous ring around mouth; n, radial nerves to each arm, ending in the eye. (From Le Conte.)

nerve cords, but they do not meet one another in a common centre of radiation. "It is difficult to see," writes Le Conte, "how such an animal can have a common consciousness," meaning thereby that co-ordinated action of all parts toward a common end could not be effected.

The mol usca present an increasingly complex organization in general and more varied and definite functions, and we find here a nervous system of increasing complexity. New parts requiring to be moved are to be found. This necessitates new ganglionic centres. In the acephalous mollusca, typified by the clam and the oyster (Figs. 6, 7), although there is no well-defined head, yet one part is distinctly the anterior portion and another the posterior. Corresponding to this distinct advance over the radiates,

there are two ante-
rior ganglia on each
side of the mouth.
These are connected

and also communi

cate with the posteri

FIG. 7.-Nervous system of an oyster.

«, cephalic ganglion; v. visceral

ganglion. (After Le Conte.)

or ganglion by means of two long lateral nerve fibres. In some cases, as in the clam, there is a ganglion in the organs of locomotion, called the pedal ganglion. The gasteropods (snails, etc., Fig. 8) and the cephalopods (cuttle-fish, squids, etc.) possess in addition distinct cephalic ganglia. These classes of animals possess much more perfect organs of locomotion and also have some well-developed sense organs, especially eyes. The articulates, including the worms, insects, etc., have a nervous system peculiarly adapted to their general structure and their activities. The locomotor apparatus is highly developed and the nervous