48 A COURSE ON ZOOLOGY. 



very important part in respiration, digestion, and circu- 

 lation. 



The spinal cord is a direct continuation of the medulla 

 oblongata, and is contained in the hollow canal of the 

 vertebral column, down which it passes as far as the 

 loins. It is composed of gray matter interiorly and 

 white matter exteriorly, a structure opposite to that 

 of the brain. The different bundles of fibres of which it 

 is made up are separated by regular grooves, and have 

 enlargements at the points where the nerves distributed 

 to the upper and lower limbs have their origin. 



The matter of the cord is very delicate. A violent 

 shock or a strong pressure upon it may occasion imme- 

 diate paralysis or even death. Such a condition happens 

 when the vertebra, especially those of the neck, are dis- 

 located, and their articular surfaces no longer exactly 

 correspond. 



The thirty-one pairs of spinal nerves that is, nerves 

 proceeding from the spinal cord are classified as eight 

 pairs of cervical, twelve dorsal, five lumbar, and six 

 sacral. These nerves have exit from the spinal column 

 through lateral openings, called intervertebral openings. 

 They have double roots, anterior and posterior. The 

 anterior roots are motor nerve-fibres, while the posterior 

 are sensory nerves. These two roots soon unite to form 

 little masses of nerve-cells, called ganglions, and from 

 the nerve that starts from the ganglions proceed the 

 branches which divide up into smaller and smaller nerves 

 as they are distributed to the extreme surface of the 

 body. 



The interlacing of nerve-branches forms what is called 

 a plexus. The first four pairs of cervical nerves form the 

 cervical plexus, supplying the neck ; the other four pairs 



