SUPPORT, MOTION, AND SENSATION 345 



distinct cephalization. The nervous system serves to correlate move- 

 ments and to give information of changes in the environment. In- 

 numerable fibers extend from an elaborate central controlling device 

 to all parts of the body. Such a nerve mechanism may be subdivided 

 into several parts. For example, in man there is a central nervous 

 system, a pcj-iphcral nervous system, and an autonomic or sympathetic 

 nervous system. 



Protective Devices for the Central Nervous System 



As this centralized ner^'ous system is the master which controls all 

 voluntary acts and indirectly all parts of the body, it is of primary 

 importance to protect so delicate a mechanism from injury. Since 

 the situation is essentially the same among the different members of 

 the large group of vertebrates, attention will be primarily directed to 

 the system as it is found in mammals, and more particularly in man. 



The skull and the A^ertebral column serve as the ''first line of 

 defense" for the all-important brain and spinal cord against possible 

 attack or injury. However, "secondary defenses" must also be 

 present. The inner surface of the skull and the vertebral column, 

 therefore, is lined with a tough membrane of fibrous connective tissue, 

 called the dura mater. Inside the dura mater the central nervous 

 system itself is also covered with a thin, closely investing membrane, 

 the pia mater, while between it and the dura mater lies the delicate 

 serous membrane known as the arachnoid. These three membranes 

 furnish additional protection to the central nervous system, but they 

 would be relatively ineffectual without the buffering effect of the cere- 

 brospinal fluid which fills the spaces between the arachnoid and pia 

 mater. Thus the vertebrate nervous system is insulated, cushioned, 

 or, to put it more graphically, furnished with "shock absorbers," that 

 enable man and other vertebrates to withstand severe shocks without 

 injury. 



Anatomy and Development of the Brain 

 The Early Development of the Central Nervous System 



Amphioxus gives Init slight evidence of an enlargement of the 

 cephalic end of the dorsal tubular nerve cord, but in the bony fishes 

 there are already five main divisions in the adult brain. These same 

 divisions are to be found in every one of the different vertebrate 

 classes, and all representative vertebrate brains have a similar embry- 



