DISEASES OF THE NERVOUS SYSTEM 111 



tal foramen) of the cranial cavitv, and terminates posteriorly in 

 the lumbar region at the upper third of that portion of the spinal 

 canal belonging to the sacrum. It is thick, white in color, irregu- 

 larly cylindrical in shape, slightly flattened above and below and 

 reaches its largest diameter in the lower cervical and lumbar 

 regions. The spinal canal is lined by the outer membrane that 

 envelops the cord, which aids in fixing this organ to the wall of 

 the canal. The spinal cord is formed by white and gray nerve 

 tissue. The gray tissue is situated within the w4iite, and it is 

 arranged in the form of two lateral comma-shaped columns con- 

 nected by a narrow commissure of gray matter. The extremities 

 of the lateral gray columns mark the origin of the superior and 

 inferior roots of the spinal nerves. The white tissue of the cord 

 is also divided into lateral portions by superior and median fis- 

 sures. The inferior fissure does not extend as far as the gray 

 commissure, leaving the lateral inferior columns connected by a 

 white commissure. There are certain centres in the spinal cord 

 that are capable of carrying on certain reflex actions independent 

 of the chief centre in the brain. The white matter of the cord 

 is made up of paths over which impulses to and from the brain 

 are transmitted. 



There are tivelve pairs of cranial nerves. Two pairs belong 

 exclusively to the special senses, smell and sight. Altogether 

 there are ten pairs that are devoted to functions connected wnth 

 the head, either as nerves of the special senses or in a motor or 

 sensory capacity (Figs. 26 and 27). There are two pairs dis- 

 tributed to other regions. These are the tenth and eleventh pairs. 

 The tenth pair or pneumogastric is distributed to the vital organs 

 lodged within the body cavities. 



There are forty-two or forty-three pairs of spinal nerves given 

 off from the spinal cord. The spinal nerves have two roots, 

 superior and inferior. The superior is the sensory root and the 

 inferior is the motor root, both uniting to form a mixed nerve 



