STRUCTURE OF NERVE-CELLS. 91 



Canada balsam without a cover-glass, and the balsam dried rapidly on a 

 warm plate. 



6. Make teased preparations from a nerve which, some days previously, 

 has been cut nearer the spinal cord. The nerve should have been prepared 

 with osmic acid, as in Lesson xviii. sec. 3. Notice the breaking up of the 

 my el in of the medullary sheath, varying in degree according to the length of 

 time the section has been made previously. In preparations from the central 

 cut end of the nerve new fibres may be seen budding from near the 

 extremities of the undegenerated fibres of the stump. 



Nerve-cells only occur in the grey matter of the nerve-centres, and 

 in little groups on the course of certain of the peripheral nerves, these 

 groups often causing nodular enlargements of the nerves, which are 

 known as ganglia. The most important ganglia are those which are 

 found upon the posterior roots of the spinal nerves, upon the roots of 

 some of the cranial nerves, and upon the trunk and principal branches 

 of the sympathetic nerve. Minute ganglia are also found very nume- 

 rously in connection with the nerves which are supplied to involuntary 

 muscular tissue, as in the heart, alimentary canal, bladder, uterus, etc. 



FIG. 108A. LONGITUDINAL SECTION THROUGH THE MIDDLE OF A GANGLION ON THE 



POSTERIOE ROOT OF ONE OF THE SACRAL NERVES OF THE DOG, AS SEEN UNDER A 

 LOW MAGNIFYING POWER. 



a, nerve-root entering the ganglion ; b, fibres leaving the ganglion to join the mixed spinal 

 nerve ; c, connective-tissue coat of the ganglion ; d, principal group of nerve-cells, with 

 fibres passing down from amongst the cells, probably to unite with the longitudinally 

 coursing nerve-fibres by "T-shaped junctions. 



Nerve-cells vary much in size and shape ; many are large, some being 

 amongst the largest cells met with in the body, but others are quite 

 small. The nucleus is generally large, clear, and spherical, with a 

 single large and distinct nucleolus ; there is also a network of chromo- 

 plasm. The shape depends a good deal on the number of processes, 

 and the manner in which they come off from the cell. If there is but 

 one process the cell is generally nearly spherical. This is the case with 



