In the fore-brain there are centres for specially governing all the motor organs; but by a strange arrangement the main root of the brain is overwhelmed by its own offshoot, the hemisphere, or lobe which gives rise to the olfactory bud. In fact, so great is the importance of the sense of smell to an animal whose one object in life is to find food, that, instead of the hemisphere being subordinate to its parent, it seems to take over most of the latter’s business, receiving a report of the sensations collected by it, and sending out orders upon its own initiative. Yet, unimposing though the history of this division of the brain may be, it ultimately becomes the seat of consciousness, whereby the mental processes are carried on, and whence all voluntary movements spring.
Diagram 62.—Showing how the Cerebral Hemispheres are developed from A, the Foremost Bulb of the Brain.
Of course, in order to do this, the hemispheres have to grow considerably, and thus we find them enveloping the rest of the fore-brain and swamping it in structure as well as in function. [Diagram 62] indicates how this is done, while [Diagram 63] shows roughly the proportion and position the different parts of the brain ultimately attain. Finally, [Diagram 64], which is rather more realistic, but still much simplified, presents a view of the organ in the head.
The size of the cerebral hemispheres, compared with the rest of the brain, is especially remarkable. So, too, is their endeavour to increase their surface still more by throwing it into deep folds. ([See Diagram 64.]) These two features vary with the position of the animal in the scale of development; in man, who stands highest in intelligence and dexterity, the hemispheres are very large indeed compared with the other organs, and seamed all over with a maze of winding furrows. Another remarkable feature is the extreme degree to which specialization is carried out. Different parts of the body are represented, each by a small area of the cortex, or surface layer, and we know at what spot on the cortex such sensations as sight and hearing are perceived, and from exactly what little patch the impulse to move each limb emanates. In the accompanying diagrams ([65] and [66]) these areas are mapped out, their locality being fixed by the principal folds which act as landmarks on the surface of the hemisphere.
Diagram 63.—Relation of Different Parts of the Brain.
There is another important fact which we must not omit to mention in speaking of this localization: each hemisphere presides over the opposite side of the body. Early in development the nerve fibres from the eye cross over to the opposite side of the brain, and the afferent fibres from the lower parts of the body have accordingly to follow suit. Then, as the efferent fibres—i.e., those which set the muscles in motion—have to bring about the movements in response to information received, they must also cross to get back to the side from which it came. So, if a tumour grows inside the head on the right side, it is the left eye which becomes sightless, or the left hand which grows numb and powerless, according to the part of the cortex which is pressed upon.
Perhaps the most interesting part of the whole body is that little band of the cortex running upwards from behind the temple to the crown of the head, in which (cf. Diagrams [64], [65] and [66]) the motor areas of the limbs, and the perception of those sensations which we have grouped together and called ‘touch,’ are situated. The minute structure of this region is roughly shown in section in [Diagram 67], as it has been made out with the microscope; but only a few of the nerve cells are shown, the connective tissue of feltwork in which they are suspended, and the bloodvessels by which they are nourished, being left out. All the structures represented are of course very, very small; the large black patches which represent cells would really be invisible, and the whole field of the diagram only a mere speck, to the naked eye.
