Protection of the body is, to a certain extent, secured by its being inclosed in a bony case, the ribs. It may be thought strange that the advantage to be derived from this defensive armor is not extended to the whole body. It very probably would be, were there not some active influence opposing it. This influence may be, the necessity of expansion of the ventral surface. Expansion and contraction in the chest are regular and rhythmical, and are secured by the jointed connection of the ribs with the breastbone. Expansion in the abdomen is irregular and at times excessive. Incasement in an inflexible rib-case would, therefore, prove highly disadvantageous.
Yet no flexible condition can well arise in response to expansions appearing irregularly and often at long intervals. As the ribs, therefore, could not gain, by selective adaptation, the proper motive relations to these occasional expansions, and as inflexible ribs would be a disadvantage, abdominal ribs have failed to appear.
The general characteristics of the body being thus necessarily as we find them, and the position, length, joints, and action of the limbs being inevitable results of their purpose, as the organs of animal motion, it remains to trace the origin of the head with its organs.
The head is simply the carrier of the organs of the special senses. The brain, so far as its secondary action is concerned, might very well be situated in any other portion of the body, but we think it can be shown that the location of the eyes governs that of the brain, and that the head with all its organs is an adjunct of the eyes.
These important organs necessarily occupy the most elevated part of the body. The outlook is better from this location, and the safety of the animal is thus more assured. An anterior location is also highly desirable, as otherwise the forward-moving animal would be in constant peril from obstacles in its path, and would be unable to perceive the prey it was pursuing. Also, as the delicate character and exposure of the eyes tend to limit their number, the portion of the body bearing them must be sufficiently flexible to permit vision in all directions.
All these requirements tend to the production of an anterior, elevated organ mounted on a neck of flexible movement, and as long as is consistent with easy support of the weight of the head. The ears, also, are best situated upon this head organ, and in such a position as to adapt them to catch sound from all directions.
And this position of the eyes and ears necessarily requires the brain, for its fullest effectiveness, to be likewise situated in the head. For the greatest safety follows the quickest warning of danger. But, as is well known, the nerves are slow in their conveyance of sensations. The animal, therefore, whose eyes and ears are nearest the brain be-
and aid to the venous current are equal. But the fact holds good in all vertebrate animals, that the quantity of blood to be driven to the brain exceeds that to be driven to the lower body.