CHAPTER X 

 NERVOUS SYSTEM, SENSES, AND SENSE ORGANS 



Nervous system: brain, spinal cord, nerves. Sensory and motor nerves. 

 Olfactory organs. Sense of smell. Eyes: structure, modifications, 

 position. Sense of sight. Auditory organs. Otoliths. Connection 

 of air-bladder with internal ear. Weberian mechanism. Sense of 

 hearing. Internal ear and equilibrium. Sense of taste. Of touch. 

 Barbels. Other tactile organs. Lateral line: structure, functions. 

 Sense of pain. Schooling or shoaling sense. Sleep. Reflex and 

 conscious actions. 



The principal organs of a fish's body have now been described, 

 and it remains to consider the nervous system, the elaborate 

 organisation of brain, nerves, and sense organs, unifying and 

 co-ordinating the complex activities of the body, and placing 

 the various parts in communication with one another and with 

 the outside world. This has been compared to a telephone 

 system with the central exchange represented by the brain; but 

 although this analogy is in many respects a good one, it must 

 not be pushed too far, for as will be shown in the following 

 pages, there are a number of important differences between 

 the two organisations. 



As in higher vertebrates, the nervous system consists of brain, 

 spinal cord, and nerves. In the newly formed embryo the 

 first two are indistinguishable, and together form a simple tube, 

 the medullary canal, lying along the upper surface of the body. 

 That part of the tube which is to form the spinal cord soon 

 becomes more solid through the thickening of its walls, but a 

 minute central canal persists throughout life as a vestige of the 

 original cavity. The anterior end of the tube in the head region 

 enlarges to form the brain, and at the same time two transverse 

 constrictions divide this into three hollow chambers or primary 

 vesicles, known respectively as the fore-, mid-, and hind-brain. 

 As development proceeds, certain parts of the walls of the 

 vesicles become variously thickened, and others give rise to 

 hollow outgrowths, which may be either median or paired. 

 In this way the elaborate brain of the adult fish comes into 

 being, the original three chambers continuing to exist as a series 

 of linked spaces or ventricles. 

 M 177 



