CHAPTER XXI 



THE CENTRAL AND PERIPHERAL ORGANS OF THE NERVE 



SYSTEM 



The nerve system has been resolved by histologic investigation into single 

 morphologic units termed neurons. Though they have a common origin 

 they have assumed different forms, in different regions of the system in the 

 course of their development. Nevertheless it is apparent from an ex- 

 amination of their structure that they have many features in common. 

 The neurons in their totality constitute the neuron or nerve tissue. 

 Arranged in both a serial and a collateral manner into a regular and con- 

 nected whole, they form the neuron or nerve system. The neurons, more- 

 over, are grouped into more or less complexly organized masses termed organs 

 which in accordance with their location may be divided into (i) central organs 

 and (2) peripheral organs. 



The Central Organs. The central organs of the nerve system are 

 the encephalon and the spinal cord, lodged within the cavity of the cranium 

 and the cavity of the spinal or vertebral column respectively. The general 

 shape of these two portions of the nerve system corresponds with that of the 

 cavities in which they are contained. The encephalon is broad and ovoid, 

 the spinal cord is narrow and elongated. 



The encephalon is subdivided by deep fissures into four distinct, though 

 closely related portions: viz., (i) the cerebrum, the large ovoid mass, occu- 

 pying the entire upper part of the cranial cavity; (2) the cerebellum, the 

 wedge-shaped portion placed beneath the posterior part of the cerebrum 

 and lodged within the cerebellar fossae of the cranium; (3) the isthmus of 

 the encephalon, the more or less pyramidal-shaped portion connecting the 

 cerebrum and cerebellum with each other and both with (4) the medulla 

 oblongata. 



The spinal cord is narrow and cylindric in shape. It occupies the spinal 

 canal as far as the second or third lumbar vertebra. 



The central organs of the nerve system are bilaterally symmetric, con- 

 sisting of distinct halves united in the median line. The cerebrum is sub- 

 divided by a deep fissure, running antero-posteriorly, into two ovoid masses 

 termed cerebral hemispheres; the cerebellum is also partially subdivided into 

 hemispheres; the isthmus likewise presents in the median line a partial 

 division into halves; the medulla oblongata and spinal cord are subdivided 

 by an anterior or ventral and a posterior or dorsal fissure into halves, a right 

 and a left. 



The Peripheral Organs. The peripheral organs of the nerve system 

 in anatomic and physiologic relation with the central organs are the ence- 

 phalic and the spinal nerves. 



The encephalic nerves, twelve in number on each side of the median line, 

 are in anatomic relation with the base of the encephalon, and because of 

 the fact that they pass through foramina in the walls of the cranium they 

 are usually termed cranial nerves. 



531 



