THE STUDY OF ORGANS. 33 



buds ; the olfactory bulbs, and touch corpus- 

 cles, by which we receive respectively the senses of 

 smell and touch, are built on a similar plan. 



When a sense-organ, as in its lowest forms, consists 

 merely of an irritable cell, its usefulness cannot be 

 very groat. To warn the animal of the presence of 

 danger or food, and to enable the animal to profit by 

 the warning, so that it may feel and act, not merely 

 shrink and glirug, communication between the different 

 parts of the body is required, and especially communi- 

 cation between its sense-cells. This is supplied by 

 nerves, which also arc originally derived from the 

 epiblast (figs. 23 and 24, p. 113). Long threads, called 

 nerve fibres, pass to peculiar large branched cells 

 called nerve- cells. A group of nerve- cells is called 

 a ganglion ; a band of nerve fibres, passing between 

 opposite or adjacent ganglia, is called a commissure ; 

 and a large number of united ganglia, united by many 

 commissures, is called a brain. The brain attains its 

 highest development in mammalia and birds. It is 

 customary to restrict the application of the term to 

 the vertebrata only ; but there is no reason why the 

 mass of ganglia contained in the head or anterior 

 region of most of the invertebrate animals should not 

 also be called a Brain. It would lie beyond the scope 

 of a little book like this to discuss at any length the 

 functions of the brain and its connected system o£ 

 nervous fibres. Briefly, we may say that its prim- 

 ary function is double : it has to receive and put 

 together messages of sensation (sensory impulses), 



D 



