THE NERVOUS SYSTEM 453 



(4) The silver method of Cajal, using the alcohol fixation (technic 2, p. 30), 

 may also be advantageously used to display many of the above details of struc- 

 ture in embr>'o chicks. It is also capricious. 



Whole neurones with long axoncs obviously cannot be directly demonstrated 

 in single sections. To demonstrate these serial sections and indirect methods 

 (degeneration, etc.) are used (see Fibre Tracts of Cord). 



PRACTICAL STUDY 



Transverse Section of Six-day Chick Embryo (Technic i, p. 452). — Using 

 a lower-power objective, first locate the cord and determine the outlines of gray 

 matter and white matter. Observe the spinal ganglia lying one on either side 

 of the cord (Fig. 316, a). At least one of the ganglia will probably show one or 

 more bipolar cells, sending one process toward the periphery, the other toward 

 the spinal cord. Note that the peripheral process is joined, beyond the ganglion, 

 by fibres which come from the ventral region of the cord (fibres of the anterior 

 root). In some specimens the latter can be traced to their origin in the cells 

 of the anterior horn (Fig. 316, d). The union of the peripheral processes of the 

 spinal ganglion cells and the anterior horn fibres is seen to make up the mixed 

 spinal nerve (Fig. 316,/). Observe the central processes of the spinal ganglion 

 cells entering the dorsal column of the cord and bifurcating (Fig. 316, b). As 

 these branches pass up and down the cord, only a short portion of each can be 

 seen in a transverse section. Note the fibres (collaterals) passing from the white 

 matter into the gray matter. The fact that they are finer than the longitudi- 

 nal fibres in the white matter shows that they are branches of the latter. 

 Even in transverse section the point at which they leave the latter may occa- 

 sionally be made out. Note in some of the sections a little round mass just 

 ventral and to the inner side of the spinal ganglion, in which nerve cells may 

 be seen, and some fibres passing into or out of it. This represents the begin- 

 ning of the sympathetic system with its chain of ganglia. Note the relation 

 which this bears to the spinal cord and spinal ganglia. 



In the same or other transverse sections study the column cells of the cord, 

 carefully distinguishing between the dendrites and axone. This is not always 

 easy, but the axone can usually be distinguished as being more slender, with 

 smoother outline and more uniform size throughout its course. Axones may 

 come off from dendrites as well as from the cell bodies. At least one tautomeric 

 and one heteromeric column cell should be found and studied (Fig. 317). Study 

 also the collaterals if they are stained. Remember that only a few of the elements 

 present are stained in Golgi preparations and that there are apt to be present 

 irregular silver precipitates without any significance. Capillaries often appear 

 as a coarse brown-stained meshwork. 



Study also any spongioblasts that may be stained. Those with nuclei near 

 the central canal give a fair representation of the ependyma cells of the adult 

 cord, except the cell does not usually in the latter extend entirely through the 

 wall of the neural tube. 



Longitudinal Section of Six-day Chick Embryo (Technic i, p. 452). — Using 

 a low-power objective locate gray matter and white matter and identify plane 



