

2 1 8 Life and Health 



Passing from the medulla, they descend near the oesophagus, 

 sending off, on their way, branches to the windpipe, the larynx, 

 the chief digestive organs, the lungs, and the heart (Fig. 125). 



The eleventh pair, the spinal accessory nerves, supply some 

 of the muscles of the neck and the back. 



The twelfth pair, the hypoglossal nerves, control the move- 

 ments of the tongue in speech and in swallowing. 



Experiment 95. If the brain has been carefully removed, most of 

 the twelve pairs of cranial nerves may be identified by careful study. 

 Such as are found may be compared with the cranial nerves indicated 

 on a diagram of the human brain. 



333. The Spinal Cord. The spinal cord is a column of 

 grayish-white, soft substance, which extends from the base 

 of the skull to the lower border of the first lumbar verte- 

 bra, where it narrows off into a slender filament. It is 

 continuous with the medulla oblongata and is lodged in the 

 canal of the spinal column. 



The spinal cord is from sixteen to eighteen inches long 

 and has about the thickness of one's little finger, weigh- 

 ing about one and one-balf ounces. Like the brain, it is 

 enclosed in three membranes, which protect the delicate 

 cord and convey vessels for its nourishment. The % space 

 between the two inner membranes contains a small quan- 

 tity of fluid, supporting the cord, as it were, in a water 

 bed. It is thus guarded against shocks. 



334. Structure of the Spinal Cord. The arrangement of 

 the parts of the spinal cord is best understood by a trans- 

 verse section. Two fissures, one behind, the other in front, 

 penetrate deeply into the cord, very nearly dividing it into 

 lateral halves. In the middle of the isthmus which joins 

 the two halves is a very minute opening, the central canal, 

 which extends the entire length of the cord^ 



The spinal cord, like the brain, consists of gray and white 

 matter, but the arrangement is different. In the brain the 



