148 



HUMAN PHYSIOLOGY 



A. SPINAL CORD. 

 Structure. 



The spinal cord is a more or less cylindrical mass of nerve 

 tissues which passes from the base of the brain down the 

 vertebral canal. It terminates in a pointed extremity 

 at the level of the 1st lumbar vertebra. There are two 

 enlargements upon it, one in the cervical region, one in the 

 lumbar region, and from these the nerves to the arms and 

 legs come off. A fine central canal runs down the middle, 



FIG. 78. Cross Section of the Spinal Cord through the second Dorsal Segment, 

 to show disposition of grey and white matter. P., Posterior Horn ; A., An- 

 terior Horn with large cells; I.L., Intermedio-lateral Horn with small cells ; 

 L.C., Lockart Clarke's Column of Cells; P.M. and P. L. , Postero-median and 

 Postero-lateral Columns; D.C., Direct Cerebellar Tract; Asc. and Desc. 

 Ant. Lat., Ascending and Descending Antero-lateral Tracts; B.B., Basis 

 Bundles; C.Pt/., Crossed Pyramidal Tract; O.Py., Direct Pyramidal Tract. 

 On opposite side tracts which degenerate upwards are marked with horizontal 

 lines ; tracts degenerating downwards with vertical lines. (After BRUCE.) 



and the two sides are almost completely separated from one 

 another by an anterior and a posterior mesial fissure (Fig. 78). 

 Each half is composed of a core of grey matter arranged in 

 two processes or horns the anterior and posterior horns 

 (A. and P.) which divide the white matter surrounding the 

 grey into a posterior, a lateral, and an anterior column. In 

 the dorsal region a lateral horn of grey matter projects into 

 the lateral column -^(I.L.). The white matter is composed of 

 white nerve fibres, the grey matter very largely of cells and 



