50 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



Thus the olfactory lobes, the larger part of the cerebral hemispheres, and the 

 dorsal and ventral parts of the diencephalon are olfactory. The lateral walls of 

 the diencephalon and part of the optic lobes are the receptive regions for vision. 

 Another part of the optic lobes and the dorsal margins of the anterior end of the 

 medulla are the centers of hearing. The general sensations from the skin of the 

 body are received into the dorsal regions of the spinal cord, which extend forward 

 to the same regions of the medulla, where similar sensations from the head also 

 enter. The dorsal and lateral regions of the medulla, therefore, together with 

 the same regions of the optic lobes and diencephalon are centers of general 

 sensations. Taste and other visceral sensations are received in the ventro- 

 lateral regions of the medulla. The ventral portions of the brain from the 

 optic lobes posteriorly and down the whole length of the cord in the ventral 

 horns of the gray matter are the regions of origin of motor impulses. 



Obviously these primary sense centers must be connected with the motor 

 nerve cells from which nerves go to the muscles in order that an appropriate 

 response may be made to the conditions in the world outside which are reported 

 to the brain. Thus all the sense centers form intricate connections with motor 

 centers for the production of motor actions. The spinal cord and part of the 

 medulla are pathways for sensations from the body below the head to reach 

 the brain and for motor impulses to reach the body muscles. They also carry 

 out many reflex actions, i.e., actions resulting from direct connections of sensory 

 impulses with motor responses without the aid of the brain. The medulla is 

 further an important center of visceral functions, such as respiration, heart- 

 beat, etc. 



A still further mechanism is, however, required. This is a mechanism for 

 the association and correlation of sensory information and for deciding between 

 simultaneous ones. Thus suppose an animal smells some food and sees an enemy 

 simultaneously. It must make a choice between the motor reactions which 

 would result from each of these sensations if they were received separately. It 

 must act "intelligently" in such situations. Such centers of correlation are 

 naturally poorly developed in simple animals and become more and more promi- 

 nent in the brain as the complexity of the animal increases. In the frog correla- 

 tion is affected mainly in the optic lobes and diencephalon and to a slight extent 

 in the cerebral hemispheres. Hence the complete removal of the cerebral 

 hemispheres in the frog is of little consequence to the animal, except that the 

 sense of smell is lost (see Holmes, p. 309). In higher vertebrates the cerebral 

 hemispheres become more and more important as the seat of correlation, 

 co-ordination, and intelligent action. 



The cerebellum is also a co-ordination center, but one not associated with 

 consciousness. It co-ordinates, reinforces, and exercises general control over 

 motor movements, including the maintenance of equilibrium. 



