I 82 



PHYSIOLOGY FOR BEGINNERS 



along its length, nerves, called the spinal nerves, are given 

 off in pairs. There are thirty-one pairs of spinal nerves. 

 The two nerves of a pair arise at the same level, one from 

 each side of the cord. Each nerve springs by two roots, 

 one, the anterior root, from the front part, and the other, the 

 posterior root, from the hinder part of its half of the cord. 

 The two roots divide the white matter of each half of the 

 cord into three portions, called, according to their position, 

 the anterior, the lateral, and the posterior columns of 

 the white matter. The anterior and posterior roots soon 

 unite to form the nerve trunk, which then passes out of the 

 spinal canal between the arch of one vertebra and that of the 

 next to be distributed to certain muscles and to a certain part 



FIG. 83. 



A, Front view of a portion of the spinal cord. On the left side of the cord the 

 anterior roots, A.K, are cut to show the posterior roots, /'.A'. 



1>, Cross section of the cord : A, anterior fissure ; /', posterior fissure ; (/, central 

 canal; C, grey matter ; IV, white matter; A.K, anterior root; /'.A", posterior 

 root ; Gn, ganglion of posterior root ; T, trunk of a spinal nerve. 



of the skin of the limbs or trunk. Just before the roots of 

 each nerve join there is a knot-like enlargement on the pos- 

 terior root. This is the ganglion of the posterior root. 

 The nerve cells of these ganglia give off one process only, 

 which by a T-shaped junction becomes continuous with a 

 nerve fibre passing through the ganglion. 



Nature of the White and Grey Matter and Origin 

 of the Nerve Boots. The white matter of the spinal cord 

 consists, with a very little intermingled connective tissue, entirely 

 of nerve fibres, the majority of which run along the length of 

 the cord. In the grey matter there are a number of similar 

 but generally fine white medullatcd nerve fibres ; these, how- 

 ever, run in various directions. There arc also a large 



