Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III FORCED MOVEMENTS When we destroy or injure the brain on one side we paralyze or weaken the muscles connected with this side. As a consequence the morphological plane of symmetry ceases to be the dynamical plane of symmetry and the animal has a tendency to move in circles instead of in a straight line. Suppose a fish swimming forward by motions of its tail fin. Normally the stroke occurs with equal energy to the right and to the left, and the rudder action of the tail is equal in both directions, but after the lesion of one side of the brain the stroke and the rudder action cease to be the same in both directions, it is weakened in one direction. Hence the animal instead of swimming in a straight line is forced to deviate continually toward one side from the straight line of locomotion. We speak in such a case of a forced motion. When we destroy the ventral portion of the left optic lobe in a shark (Scyllium canicula), the fish no longer swims in straight lines but in circles to the right (when the right optic lobe is destroyed it swims in circles to the left). After the destruction of the left optic lobe, the muscles on the left side of the tail are weakened or semi- paralyzed, and they no longer produce the same rudder action as the muscles on the right side. Hence the impulses (or nerve processes) which flow in equal intensity to the muscles on both sides will no longer produce equally energetic rudder action of the tail to the right and to tho left, but the muscles turning the tail to the right will contract more powerfully than those turning it in the opposite direction. The outcome of this greater rudder action of the tail when moving to the right is that the fish instead of swimming in a straight line moves in a circle to the right.290 It is often the c...