I 90 VERTEBRATES 



of ganglia and nerve threads, is essentially the same as in 

 all of the other animals studied. The ganglion in the head 

 is called the brain, and is supposed to be the thought center, 

 but is not always the largest ganglion in the invertebrates. 

 The ganglia, composed of gray matter, are supposed to pro- 

 duce on irritation a certain nerve force which is conveyed 

 along the nerve threads like electricity along the telegraph 

 wire. In the higher vertebrates if the irritant is without the 

 nervous system, and more especially if it is without the body, 

 the force which originates in the small granglia is sent to- 

 ward the brain, and if it reaches it, produces a sensation. If 

 the nerve force is originated within the brain, as in thinking, 

 the force, if carried outward by the nerves to the muscles, 

 produces motion. Hence we see the connection between 

 sensation and motion. This is conscious activity. 



(b) However, there are many activities that do not de- 

 pend upon the brain, but are carried on by the nerve cen- 

 ters below the brain, the principal one of which is the 

 medulla. The beating of the heart, the digestion of food, 

 and the secretion of fluids by glands are of this class. All 

 of the processes which are connected directly with the sus- 

 tenance of life are carried on without the brain, and in a 

 manner in which the animal is unconscious. There are 

 many kinds of unconscious activity some of which are 

 partly conscious. Do all animals have conscious activity? 

 Do all have unconscious activity? 



V. Reproduction. — (i) The pigeon is reproduced 

 from eggs which are deposited in a nest carefully built in 

 some secure place, and are cared for by the mother pigeon 

 until they are hatched. The eggs of a hen will answer the 

 purpose for study quite as well as those of a pigeon. 



