THE LEECHES OF MINNESOTA 41 



nerve chain of ganglia, each connected with the ganglion preceding 

 and with the ganglion following- by two large lateral commissures 

 and a third, smaller, central commissure, the so called Nerve of 

 Faivre. The caudal portion is the posterior ganglionic mass, situ- 

 ated in the anterior side of the posterior sucker. 



The Ventral Nerve Chain. 



The ventral nerve chain is made up of twenty-one somatic 

 ganglia with their connecting commissures. Throughout the middle 

 portion of the body these ganglia are about equal distances apart, 

 but toward the ends they are crowded closer together. The last 

 three ganglia of the chain are especially close together but are 

 separated from the posterior ganglion by a much longer space, prob- 

 ably due to the pressure exerted around this region by the tissues 

 of the host. The first central ganglion is closely approximated to 

 the sub-oesophageal mass and the fifth and sixth (ganglia XI and 

 XII) are closer together than the others near them. 



The typical ventral ganglion, which may be taken as the unit 

 of structure of the central nervous system, consists primarily of a 

 fibrous portion and a cellular portion. The fibrous portion is made 

 up largely of nerve fibres entering from the connecting commissures, 

 the somatic nerves, and the six cell packets. Fibres continuing 

 through the ganglion from the lateral commissures form two lateral 

 fibre tracts and the fibres crossing from one side to the other form 

 two transverse fibre tracts, one before and one behind the center of 

 the ganglion. Among the fibres are to be found occasional leuco- 

 cytes and two large glia cells (mediane Sternzellen of Apathy, '97). 

 These glia cells are medially situated beneath the two transverse 

 fibre tracts, one anterior and one posterior to the centre of the 

 ganglion. In a few cases I have found the anterior cell divided so 

 that there were three median glia cells in the ganglion instead of 

 two. Herman, '75, described these cells as "mediane Ganglienzel- 

 len", Retzius, '91, as "kolossale Ganglionzellen" and "Nervenzellen". 

 Apathy, '97, was the first to describe them unmistakably as glia 

 cells. 



The cellular portion of the ganglion consists of six cell packets, 

 each containing nerve and glia cells, and surrounded by a regular 

 capsule from which emerge the nerve fibres of the enclosed nerve 

 cells. These packets occupy a definite position in each ventral 

 ganglion, two being on each side, lateral to the central fibrous mass, 



