48 ESSENTIALS OF PHYSIOLOGY. 



In front of the grey commissure, and uniting the white matter of the 

 two halves of the spinal cord, is the white commissure. 



The white matter is subdivided by the posterior horn of grey 

 matter into an antero-lateral and posterior column (funiculus). The 

 former is again roughly subdivided by the bundles which form the 

 anterior nerve roots into an anterior and a lateral column. 



THE REGIONS OF THE SPINAL CORD. 



Three regions are distinguished in the spinal cord, the cervical, 

 thoracic, and lumbar, each of which possesses definite structural 

 characteristics. The cervical and lumbar regions exhibit enlargements 

 corresponding with the outHow of nerves to the arm and leg respectively. 

 The differences in structure between the three regions are best seen 

 by a comparison of transverse sections. The cervical region is oval in 

 section, the long axis of the oval lying transversely, its anterior median 

 fissure is relatively shallow, and the central canal is in front of the true 

 centre of the cord. The white matter is large in amount, the anterior 

 horn (column) of grey matter is broad, and there is a well-marked 

 septum subdividing the posterior column (funipulus) of white matter 

 into medial and lateral portions. The (iH^Tregion is cylindrical, its 

 anterior median fissure is deeper, and the central canal is centrally 

 placed. The anterior and posterior horns of grey matter are both 

 narrow, and each grey crescent shows a projection in its concavity, 

 known as the lateral horn. At the base of each posterior horn of grey 

 matter, towards its medial aspect, is a special column of nerve cells, 

 called Clarke's column (the dorsal nucleus). The lumbar region re- 

 sembles the thoracic region in shape, depth of anterior median fissure, 

 and position of central canal, but its white matter is absolutely and 

 relatively smaller in amount, and both anterior and posterior horns of 

 grey matter are broad in section. Generally speaking, the white matter 

 diminishes progressively in amount from above downwards, and the 

 grey matter is most abundant in the regions from which the outflow of 

 the nerves to the limbs takes place. 



THE NERVE CELLS OF THE SPINAL CORD. 



The nerve cells jn the grey matter are for the most part irregular 

 in shape, but those in Clarke's column are somewhat fusiform, with 

 their long axes in the long axis of the spinal cord. The cells in the 

 anterior horn are larger in size than those in the posterior horn. The 

 axons of the former emerge from the anterior surface of the spinal cord 

 in several groups, which unite to form the anterior root of a spinal 

 nerve. The axons of the cells of the posterior horn fall into two groups : 



