374 THE NERVOUS SYSTEM. 



origin in the gray substance of the left side, and those emerging from 

 the left side have their origin on the right, The only uncertainty in 

 this respect is whether the decussation be complete or partial ; that is, 

 whether all the fibres of a given nerve root be connected with the 

 opposite side of the central mass, or whether a part of them originate 

 from the same, and a part from the opposite side. In a large number 

 of instances the decussation is anatomically demonstrated ; in others 

 it is inferred from the results of experiment. But there can be no 

 doubt that it is a general feature in the arrangement of the cerebro- 

 spinal system. 



The spinal cord is the simplest and most fundamental part of this 

 system. It is a nearly cylindrical nervous mass, extending from its 

 junction with the brain above to its inferior termination at the level 

 of the second lumbar vertebra. A transverse section shows that it 



is incompletely divided into 



FIG. 97. right and left lateral halves 



by anterior and posterior me- 

 dian fissures ; of which the 

 anterior is the wider and shal- 

 lower, while the posterior is 

 narrower and deeper. The 

 interior of the cord consists 

 of gray substance, in the form 

 of a double crescentic-shaped 

 mass, with the concavities of 



TRANSVERSE SECTION OK THK SPINAL CORB.-, b. the CTCSCeiltS turned Outward. ! 

 Spinal nerves of right and left sides, d. Origin of As these maSSCS are found at 



f POStei ' l r r00t ' Ua "' all parts of the cord, they are 

 in reality elongated bands of 

 gray substance, one on each side of the cord, running continuously 

 throughout its length. They are united with each other by a trans- 

 verse band of gray substance, containing nerve fibres, and known as 

 the "gray commissure," in the centre of which is a narrow longi- 

 tudinal canal, the "central canal," about 0.2 millimetre in diameter, 

 lined with epithelium. 



The anterior and posterior portions of gray substance, in each lateral 

 half of the cord, are called the anterior and posterior horns. Imme- 

 diately in front of the gray commissure is a band of white substance, 

 the " white commissure " of the cord. 



The spinal nerves are given off from the cord at regular intervals 

 in symmetrical pairs ; and are distributed to the integument and muscles 

 of the corresponding regions. In fish and serpents, where locomotion 

 is performed by lateral flexion of the spinal column, the cord is nearly 

 uniform in size, or tapers gradually to its posterior extremity. But 

 in birds and quadrupeds, where there are special organs of locomo- 

 tion, as fore and hind legs, or legs and wings, the cord is increased in 

 size where the nerves of these organs are given off. In man, the lower 



