CHAPTER IV 

 THE FROG RAN A TEM FOR ARIA (continued) 



Nervous System and Sense Organs -Ductless Glands- Life History Animals 

 and Plants Classification. 



Nervous System and Sense Organs. 



The nervous system and organs of the senses are so inti- 

 mately related that they may be regarded as forming one system, 

 whose main function is the appreciation of messages from the out- 

 side world and meeting them in the proper way. For convenience 

 of description, we may subdivide them into the Central Nervous 

 System, the Peripheral Nervous System, and the Sense Organs. 



The Central Nervous System is composed of two parts, the brain, 

 lodged within the skull, and the spinal cord, which is contained in 

 the neural canal, formed by the neural arches of the vertebrae. 

 The peripheral nervous system comprises three groups of structures : 

 nerves given off from and going to the brain, i.e. the Cranial Nerves, 

 similar nerves, the Spinal Nerves, related to the spinal cord, and the 

 Sympathetic or Involuntary Nervous System, a double chain of small 

 nerve centres lying in the ccelom close to the vertebral column. 

 The sense organs are the olfactory organ, or organ of smell, the eye, 

 the ear, and the organs of taste and touch. 



The tissue composing the central nervous system is soft, but is 

 of vital importance to the animal, and in consequence we find that 

 it is not only enclosed within the bony axial skeleton, but also, inside 

 that again, it is protected by two membranes, the meninges. The 

 outermost of the membranes, the dura mater, is tough and pigmented, 

 and applied fairly closely to the inner side of the bones of the skull 

 and vertebral column. The inner membrane, the pia mater, corre- 

 sponds to the pia mater together with the arachnoid tissue, which 

 forms an intermediate layer in the higher vertebrates. It is richly 

 supplied with blood-vessels, and attached to the surface of the central 

 nervous system, the outline of which it follows closely, dipping down 

 into all the folds. Where it covers the parts of the brain known as 

 the optic lobes it is deeply pigmented. 



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