302 MICROSCOPIC ANATOMY OF Till-: ORGANS. 



We recognize two types of ganglia : i\\z. spinal ganglion type 

 and the sympathetic ganglion type. 



The spinal ganglia contain in the lower vertebrates (fishes) 

 and in the embryos of higher vertebrates bipolar cells ; while 

 in the adult of the latter class the cells are almost all unipolar. 

 The cell body is usually large (40-70 a in diameter), and con- 

 tains a vesicular nucleus with a distinct nucleolus (Fig. 226). 

 Yellowish-brown pigment granules are also often found. 

 There is always present a nucleated capsule around the cells, 

 which is probably only a continuation of Schwann's sheath. 

 It is made up of a single layer of flat connective-tissue cells 

 (Fig. 226). 



The relations of the processes of these cells and their 

 branches have been investigated in recent years by Ramon y 

 Cajal, Dogiel, and others. According to the results of this 

 work, we can distinguish in the spinal ganglion two kinds of 

 ganglion cells : one in which the cell process divides like the 

 letter T or Y into two or three branches, which run in oppo- 

 site directions. These branches are medullated, and run for 

 some distance outside the ganglion. This cell belongs to type 

 I. The cell of type II., whose process breaks up into numerous 

 branches, is confined to the ganglion. None of the branches 

 extends beyond its limits. They break up, on the con- 

 trary, into a plexus which surrounds the nucleated capsule of 

 the cells of type I. From this plexus fine branches break 

 through the capsule and surround the cell itself (pericellular 

 plexus}. 



One cell of type II. is related usually to many cells of 

 type I. ; and many cells of type II. take part in the formation 

 of the plexus around each cell of type I. 



Besides these nerve elements already described, there are 

 present in the spinal ganglia, endings of nerve fibres arising 

 in sympathetic ganglia. These fibres break np into fine branches, 

 which surround the cells, penetrate the capsule, and give rise 

 to a pericellular network. These sympathetic fibres are related 

 especially to cells of type II., and by means of them to cells 

 of type I. 



