190 PRACTICAL MICROSCOPY. 



formation of the nearest approach afforded by the cord of nerve 

 bundles.) 



5. The gray substance, (a) The central canal. (The canal is 

 lined with columnar ciliated cells in single layer. The cilia are rarely 

 demonstrable in the human cord, on account of changes which occur 

 very quickly after death. The canal is usually broadest in its lateral 

 diameter, viz., at upper portion of cervical region, from one-one 

 hundredth to one-two hundredth of an inch, (b) The ground sub- 

 stance. (This consists, 1st, of exceedingly minute fibres, formed by 

 the repeated subdivision of the axis cylinders the primitive fibrillae ; 

 2d, of the delicate neuroglia fibres. It is usually difficult in a 

 section to differentiate between the two. Attempts have been made, 

 with more or less success, to differentiate by means of staining agents.) 

 (c) Large ganglion cells. (Select a field in the anterior horn. 

 The straight, unbranching axis-cylinder process can frequently be 

 distinguished. Note the large, shining nucleus and the deeply stained 

 nucleolus. These cells are frequently deeply pigmented.) (d) Small 

 ganglion cells. (Best seen in the posterior horn. In the dorsal 

 cord a collection of medium-sized cells presents, just within the poste- 

 rior commissure and encroaching upon the white substance posteriorly 

 to this, the column of Lockhart Clarke.) (e) Lymph spaces. (Ob- 

 served as a somewhat clear space around the ganglion cells.) (f) 

 Blood-vessels. (These are much more numerous here than in the 

 white portion; and arteries may frequently be found of considerable 

 size.) (g) Peri-vascular lymphatics. (Find an artery in trans- 

 verse section, and observe the clear space around it, which may be 

 mistaken for the result of contraction of the tissue in hardening. 

 Careful study will reveal minute branches of cells, passing between 

 the adventitia of the blood-vessel and the wall of the lymph space.) 



