NEUROLOGY 



The Nervous System. — The nervous system is an apparatus by 

 means of which animals appreciate and become influenced by im- 

 pressions from the outer world. Animals react on these impressions, 

 and thus are enabled to adapt themselves to their environment. 

 This system is the organic substratum of hfe, sensation, and motion. 

 Broadly stated, the nervous system connects the various parts of 

 the body with each other, and to coordinate the parts into a har- 

 monious whole in order to carry on the bodily f imctions methodically 

 and to control the physiological division of labor throughout the 

 organism. 



The nervous system consists of two parts. The first is the cerebro- 

 spinal system, which comprises the central nervous axis, including 

 the brain and the spinal cord, and the peripheral nerves, including 

 the cranial and the spinal nerves. The second is the sympathetic 

 nervous system. The two parts of the system are closely linked 

 together, and both terminate in peripheral nerve endings, including 

 those of special sense, of sensation, and of motion. 



The cerebrospinal nerves especially preside over the special senses, 

 motion and sensation; and the sympathetic over the digestive, 

 the pulmonary, and the vascular apparatus. 



From a structural standpoint, the nerve system consists of cell 

 elements pecuharly differentiated from all other tissue cells in that 

 their protoplasm is extended in the form of processes, often to great 

 distances from the nuclear region. The cell elements are held in 

 place by supporting tissue and receive an abundant blood supply; 

 they are partly of ectodermal and partly of mesodermal origin. 



The cell element of the nerve system, called a neurone, is the 

 developmental, structural, and functional unit of the, nervous 

 system. It is a single cell presenting unusual structural modifica- 

 tions. It comprises not only the nerve cell body with its numerous 

 protoplasmic processes, or dendrites, but also the axone, which may 

 vary in length from a fraction of a milUmeter to fully half the bird's 

 length. The bulk of the axone is many times the bulk of the cell 

 body. 



264 



