212 NERVOUS SYSTEM OF VERTEBRATES. 



but it has been shown that occasional dendrites pass along a 

 sympathetic nerve and reach another ganglion, in which they 

 break up into end-branches. It appears that the dendrites of 

 sympathetic cells play a minor part hi the reception of impulses 

 and it is not clear that either the pericellular plexuses of dendrites 

 or the passage of dendrites from one ganglion to another has any 

 functional significance. The neurite of the sympathetic cell 

 arises either from the cell-body or from a dendrite and may or 

 may not become myelinated. When it is myelinated it is so fine 

 as still to be distinguished from the smallest fibers of cerebro- 

 spinal origin and the myelin sheath may extend for a longer or 

 shorter part of the course of the fiber. The neurites, after a longer 

 or shorter course in the splanchnic nerves or by way of the gray 

 rami communicantes and one of the peripheral rami of the spinal 

 or cranial nerves, end in (a) involuntary muscle, (b) heart muscle, 

 (c) glands, or (d) other sympathetic ganglia. All smooth muscle, 

 whether in the wall of the alimentary canal, in the ducts of glands, 

 in the urinogenital system, in blood vessels, the skin or the eye 

 is innervated by neurites from sympathetic cells. The ending is 

 by means of simple branches often with small knobs or enlarge- 

 ments. The heart muscle is innervated by neurites from the 

 cells in the intrinsic sympathetic ganglia of the heart. The 

 endings may be more complex, somewhat like those in striated 

 muscle. The secreting cells of glands are innervated by simple 

 endings of sympathetic neurites which enter the glands along 

 the ducts or blood vessels or which come from neurones situated 

 in the glands themselves. The ending of the neurites of sympa- 

 thetic cells in other sympathetic ganglia is still a matter of dispute. 

 Histologists have described the endings of neurites upon the 

 dendrites of sympathetic cells, but physiologists have obtained 

 no functional evidence to corroborate the supposition that these 

 are the endings of neurites arising from sympathetic neurones. 

 Inasmuch as the efferent cerebro-spinal fibers are universally 

 believed to end in the pericellular baskets within the capsules of 

 sympathetic cells, it is probable that the endings in connection with 

 the dendrites come from sympathetic cells and that suitable forms 

 of experiment for determining their functions have not yet been 



