114 HISTOLOGY. 



is unknown. Cells of the third type (3) are few in the large ganglia and 

 are not found in small ones. They have long dendrites which form a "general 

 peripheral plexus" but do not extend beyond the limits of the ganglion. 

 Their neuraxons enter the sympathetic nerves as non-medullated fibers, 

 the destination of which is unknown. Sympathetic ganglia contain also 

 stellate connective tissue cells, and chromaffine cells to be considered 

 presently. The ganglia may be traversed by sensory medullated fibers 

 to lamellar corpuscles, and by medullated motor fibers which lose their 

 myelin sheaths and have non-medullated collateral branches. The motor 

 fibers and their collaterals terminate in rather coarse pericellular ramifi- 

 cations about the sympathetic cells of the motor type. There are other 

 nerve fibers, non-medullated and varicose, which form pericapsular plexuses, 

 and these are considered to be branches of sympathetic cells. 



Par a ganglia are masses or cords of cells which originate in the em- 

 bryonic sympathetic ganglia, and are characterized by being colored 

 yellowish brown by preserving fluids containing chromic acid or chromium 

 salts. The cells are therefore called chromaffine (meaning that they have 

 an affinity for chromium, and not, like 'chromatic material,' for coloring 

 matters generally). The paraganglia are either closely or slightly con- 

 nected with the sympathetic nerves. In the latter case they are applied 

 to large vessels, and in the fetus, between the branches of the spermatic 

 vessels, to the paroophoron and paradidymis. The glomus caroticum at 

 the bifurcation of the carotid artery, and the glomus coccygeum associated 

 with the median sacral artery, are knots of vessels both of which contain 

 clumps of chromaffine cells. The organs discovered by Zuckerkandl at 

 the origin of the inferior mesenteric artery may be classed with them. 

 Single chromaffine cells, or small groups of them, occur diffusely in the 

 sympathetic ganglia and nerves. The entire medulla of the suprarenal 

 gland in the higher vertebrates is composed of them. Since the extract 

 of such cells, on intravenous injection, causes a marked increase in the 

 blood pressure, the chromaffine cells are considered to secrete into the 

 blood a specific substance w T hich maintains the normal tonus of the vessel 

 walls. 



SPINAL CORD (Medulla spinalis). Development. The early develop- 

 ment of the medullary tube has been shown in Fig. 109, p. 92. The 

 tube at first consists of separate cells but these soon unite to form a syncy- 

 tium. Those nuclei of the syncytium which border upon the central 

 canal divide repeatedly by mitosis and many of them are forced outward 

 radially. The protoplasm of the syncytium increases more rapidly than 

 the nuclei, and forms a non-nucleated network at the periphery of the 

 tube; this is the white layer [sometimes called mantle layer]. The fibers 



