8o THE MICROSCOPE. 



must ask you now to follow me in the discussion of a system which 

 has to do with sensation and volition, and which is not represented 

 in the plant. 



5. The nervous system. — Let me draw on the black-board an 

 insect with its wings extended. You see its body consists of three 

 parts, the abdomen, the thorax and the head. On each side of the 

 abdomen is a row of spiracles or breathing holes, for insects never 

 breathe at the mouth. From the thorax project the legs and the 

 wings. At the head we find the eyes, the tentacles and the palpi. 

 I will now draw a line through the middle, dividing 'the head, thorax 

 and abdomen into equal parts. This line is a great nerve cord. I 

 now make on this cord a triple series of knots, one in the abdomen^ 

 one in the thorax and one in the head. These are the nerve ganglia 

 or brain masses. This represents the type of the insect nerve sys- 

 tem. The system in effect has three brains, as each set of ganglia 

 is functionally one brain. Let us now on the board diagram a 

 human brain. You see it is in three masses. The forward and 

 largest one is the cerebrum, which serves the functions of sensation 

 and volition. The smaller one back of the head is the cerebellum, 

 which serves the functions of respiration, while this, the smallest, 

 and placed under the others, is the medulla oblongata, and is set for 

 the functions of locomotion. You notice that the spinal cord is 

 united to the mass, especially to the medulla oblongata, which in- 

 deed is by some regarded in the light of an expansion or enlarge- 

 ment of the spinal cord. Let us compare this human nervous sys- 

 tem with that of the insect. These ganglia making the brain mass 

 in the head of the insect are set for the functions of these organs of 

 the senses, the tentacles, the palpi and the eyes, and what of intel- 

 lectual power the little thing may have. This thoracic brain mass 

 is for the functions of these legs and wings, that is, for locomotion. 

 This abdominal nerve mass is for the functions of these two rows of 

 spiracles, and the wonderfully complicated apparatus of branchial 

 tubes thus connected — that is, these ganglia serve the purpose of 

 respiration. 



The typical nerve system of a mollusc is not arranged upon a 

 straight line as is that of an insect, but as it were upon a circular 

 cord, which can vary greatly as the sides are brought towards each 

 other, thus making a more or less compressed, or narrow ellipse. 

 On this chart is a diagram or plan of the nervous system of the 



