THE NERVOUS SYSTEM. II 



The teats or mammeB from which this class derives its technical name, 

 are supplied with the milk which supports the young by glands con- 

 sisting of bunches of tiny cells. These by means of very small tubes pour 

 their secretions into larger vessels, which unite into five or six principal 

 vessels, that are capable of enlargement according to the amount of 

 milk which they are called upon to hold. These reservoirs become 

 smaller towards the mamma or teat, and serve as tubes for the convey- 

 ance of the milk into the mouth of the young. 



As regards the organs of sense, that of sight is perhaps more highly 

 developed in birds than in any other class of animals, but the others are 

 generally most highly developed in the Mammalia. Especially is this the 

 case with the sense of touch, which reaches its highest delicacy in the 

 human hand. The sense of taste varies according as the animal is 

 Herbivorous, Carnivorous, or Insectivorous. 



These various senses convey intelligence of the external world to the 

 brain, and from the brain the voluntary muscles receive their orders to 

 exercise their various functions. The brain which receives this intelli- 

 gence and issues these directions, together with the multitudinous chan- 

 nels through which they are conveyed, belongs to the Nervous System. 

 From the great nerve mass, the brain, protected by the bony armor of the 

 skull, there runs the great cord of nervous matter, the spinal cord, which, 

 defended by the vertebra, extends along the back, giving out branches 

 of various sizes. These nerves are composed of fibres, and those fibres 

 which form the anterior root of a nerve give rise to motion, and those 

 which form the posterior root give rise to sensation. This nervous sys- 

 tem, then, not only enables us to move our bodies and to know what 

 is going on in the external world, but enables us to discriminate nutri- 

 tious from innutritions matters, tells us when food is needed, gives us 

 the power to seize and kill, guides the hand to the mouth or the mouth 

 to the food, and governs all the movements of the jaws and of the 

 alimentary canal — it rules the vital actions. 



The brain varies considerably in size ; in some Mammalia it resembles 

 the brain of birds, but as we rise in the scale it quickly changes from the 

 less to the more perfect, and displays convolutions which in number and 

 extent are proportionate to the intelligence of the animal. 



The intelligence of man and the intelligence of the brute creation 

 have been distinguished by the names oi Reason and Instinet respectively. 

 A distinction like this is convenient, and unobjectionable if we remem- 



