222 OUR BODIES AND HOW WE LIVE 



or consciousness. It also sends out, by means of its motor 

 nerves, the commands of the brain to the voluntary muscles. 



Experiment 66. To show the spinal cord. Get at the market an 

 uninjured piece of the spine of an ox. Cut this across with a sharp 

 knife, and examine it for the following points. The cord is clothed 

 with a vascular membrane, the pia mater, and is composed partly of 

 a white substance lying on the outside, and partly of a pinkish-gray 

 substance lying within. There are two crescentlike masses of gray 

 substance (arranged roughly in the form of an H), lying one in each 

 half of the cord and joined by a narrow bridge of the same material, 

 which crosses the middle of the cord. The white matter surrounds 

 the gray crescents. 



The cord is almost divided into halves (exactly similar to each 

 other) by an anterior and posterior fissure. In the middle of the 

 bridge of gray matter there is a little canal, called the " central canal," 

 which runs the whole length of the spinal cord. This canal cannot, 

 however, be seen with the naked eye. 



299. Reflex Action of the Cord and Brain. The spinal 

 cord is not merely a bundle of nerve fibers for carrying 

 messages to and from the brain. It also acts as a kind of 

 independent center, receiving messages from certain parts 

 of the body by means of its sensory nerves, and on its 

 own authority sending back orders to the muscles by its 

 motor nerves, without waiting to consult the brain. This 

 is known as reflex action. 



If one is asleep, and the feet are gently tickled, the legs 

 will be moved out of the way without the sleeper necessarily 

 being awakened. When the spine is broken by an injury, 

 causing pressure upon the cord, all sensation and motion are 

 lost in the paralyzed limbs. But if these paralyzed limbs 

 are irritated, as by pricking the soles of the feet with a needle, 

 then the legs kick out vigorously. The injured person does 

 not feel the pain of the needle, and can exercise no control 

 over the legs. There is no conscious action whatever. 



