HISTOLOGY OF SENSORY GANGLIA OF BIRDS 257 



1. Spinal ganglia 



In spinal ganglia, the nerve cells at the periphery lie close to- 

 gether, while in the central part they are separated into groups of 

 4 to 8 cells by bundles of processes, the fibers from the peripheral 

 cells uniting with those from the central cells to form the separat- 

 ing bundles. Within the groups the cells are arranged in single 

 rows lying parallel to the long axes of the ganglia. The cells 

 are also slightly closer together at the proximal than at the distal 

 end because the centrally directed processes are uniformly smaller 

 than those peripherally directed. 



The spinal ganglion cells of the chick vary in size, the major 

 diameters ranging between 22 and 75 /x and the minor between 18 

 and 40 fx. There is also a variation in the shape of the cells, the 

 more frequent forms being rounded (figs. 1, 4), elliptical (figs. 

 6, 7), and pear-shaped (fig. 3), but others in close contact with 

 surrounding cells are irregularly polyhedral. 



. The nuclei of the ganglion cells are sharply defined and vary 

 from round to elliptical in shape. The rounded average about 

 8 M in diameter and the elliptical average 9 to 10 ^ in length and 

 8 /i in breadth. The nucleus is usually centrally located (figs. 

 1, 4), but in some cases it is close to the cell wall (fig. 3). The 

 long diameter of the nucleus while generally parallel to the major 

 axis of the cell, is sometimes at right angles to it (fig. 5). The 

 larger nuclei are found in the larger cells, but measurements show 

 that the size of the nucleus does not vary in proportion to the 

 increase in the size of the cell, A single centrally located nucleo- 

 lus is usually to be seen. 



Each cell is enclosed in a thin connective-tissue capsule or 

 mantle that extends a greater or less distance along the process 

 (figs. 1, 6). A considerable space separates this mantle from the 

 wall of the cell. The round to elliptical capsular nuclei vary in 

 size and contain one or two centrally located nucleoli. 



About 98 per cent of the spinal ganglion cells of the adult chick 

 are unipolar and the processes mostly without glomeruli. A 

 small percentage of the processes had an initial glomerulus of 

 simple character (fig. 7) showing one of the most comphcated 



