6. The Perpendicular Fasciculus (Fasciculus Perpendicularis, Fig. 98).—This is a very broad vertical system located just in front of the occipital pole. It extends from the inferor parietal and superior occipital gyri, above, down to the middle and in ferior temporal, the lateral occipital and the fusiform gyri. It is often classed with the short association fibers. Its function is doubtful.
The fore-brain is best adapted to the development of the intellectual faculties, as Johnston has pointed out: z. It possesses a correlation center free from the dominance of any one set of impulses. The reduction of the nervus terminalis to a functionless 'remnant provides this indifferent nucleus, and within it all varieties of impulses from every part of the body may meet on an equality and interact upon one another.
2. The fore-brain is farthest removed from the points of stimulation, being connected with them only by long chains of three or more neurones. The long arcs connecting it with the periphery remove the fore-brain from the field of simple reflexes, because of the long reaction time necessitated by such long arcs. They also winnow out the impulses at each synapsis and at each successive junction of the afferent and efferent limbs of the various arcs; so that only the more powerful impulses, those having a broad general significance to the organism, and the impulses for which the way is opened by expectant attention succeed in entering the correlation centers of the fore-brain. A calm interaction and balancing of impulses and a rational response are thus rendered possible in the fore-brain, as it is in no other part of the brain. These correlation centers of the fore-brain, which develop into the marvelous organs of memory, judgment and reasoning, are the full fruition of their diminutive primordia laid down in the lowest fishes (Anat. Rec., Vol. 4).
Evolution of the Brain.—In the very simple animal forms (as the coelen terates) the nervous sytem is represented by single reflex mechanisms.
These mechanisms at first are complete in a single sense-cell; but later they are formed by the junction of an afferent and an efferent neurone. They are perfected and elaborated by the development of intermediate links between the afferent and efferent neurones, and the reflex mechanisms are multiplied with little or no interconnection between them; hence invertebrates are reflex organisms. Vertebrates present a series of seg mental reflex arcs in the neural tube which become correlated with each other, step by step; association tracts of increasing length are built up for this purpose; the reflex arcs are elongated and their efferent limbs form a common motor path for the mechanism of locomotion. Intersegmental correlation progresses in complexity toward the head of the organims. With this progressive process the correlation neurones establish a metre general connection with all parts of the organism, the impulses received become more varied and the reflex mechanisms assume a wider and more general significance. As a result, a correlation center of great importance is built up in the posterior nuclei of the medulla (Coghill). The progres sively more elaborate intersegmental correlations of the cephalic mechan isms gradually leads to local hypertrophy and the evolution of the brain. The elaboration of a correlation center for the muscle and static senses forms the cerebellum. The building up of the more complex and extensive correlations, the correlations that cohnect and harmonize all the nervous mechanisms of the organism, somatic and visceral, forms the cerebrum with its massive hemispheres. The development of the brain is largely determined by the imperative demands of food-getting and the consequent formation in the head of the organs of special sense (J. B. Johnston).