512 APPLIED BIOLOGY 



any backbone which may be obtained at a meat-market 

 will show that the cord lies dorsal to the central axis. More- 

 over, the cord is not completely covered by bone, and at 

 the uncovered places are the spinal nerves. These are ar- 

 ranged in pairs (thirty-one pairs altogether). Each nerve 

 is divided near the cord, and one branch or root joins 

 the dorsal side of the cord, while the other joins the ven- 



tral side. On each 

 dorsal root is a 

 thickening called 

 spinal ganglion, 

 which contains 

 nerve-cells whose 



FIG. 162. Diagram showing relations of a cross . 



section of spinal cord to spinal nerves. The fibers extend into 



X-shaped central mass is gray matter, sur- the COrd and also out 



rounded by white matter. PF, posterior or dor- , , , 



sal fissure ; AF, anterior or ventral fissure ; PR, through the nerVCS 



dorsal root ; AR, ventral root of spinal nerve (Fig 162) The Cells 



fe);0, spinal ganglion. (From Huxley-Lee.) ^^ fibers ^^ 



tute the ventral root lie inside the cord. 



A cross section of a spinal cord shows an X- or H-shaped 

 figure in the center. In a fresh cord this is silvery gray, 

 and is called the gray matter (Fig. 162). The surrounding 

 whitish tissue (white matter) is chiefly composed of con- 

 nective tissue and covered nerve-fibers. Many of these 

 fibers extend to the brain and others to the regions of the 

 cord where other spinal nerves are attached. The gray 

 matter contains many nerve-cells, especially those whose 

 fibers extend out through the ventral root into the spinal 

 nerve and thence to muscles and other organs. 



The spinal cord is surrounded by membranes which sepa- 

 rate it from the hard walls of the backbone. These con- 

 tain numerous blood-vessels, some of which extend into the 

 cord. Similar membranes are between the brain and the 

 skull. 



Deep furrows or fissures extend longitudinally on both 



