266 



THE ANIMAL PARASITES OF MAN 



Exp. 



sp.. 



extending anteriorly as well as posteriorly. The remarkably small 

 number of fibres, as well as ganglion cells, is characteristic of 



the nervous system of all Nema- 

 todes. Immediately behind the 

 pharyngeal ring (fig. 185, Lg.) an 

 agglomeration of ganglion cells lies 

 at either side (lateral ganglia) ; 

 part of their off-shoots form the 

 pharyngeal ring, and part are di- 

 rected posteriorly and ventrally, 

 and unite partly in front of and 

 partly at the back of the excretory 

 pore, with fibres originating direct 

 from the pharyngeal ring, and pass- 

 ing along the ventral median ridge 

 to the back, these fibres then to- 

 gether form the ventral median 

 nerve (fig. 185 V.m.n.). This nerve, 

 originally consisting of thirty to 

 fifty fibres, becomes attenuated in 

 the female quite evenly in its 

 further course. There is also an 

 agglomeration of ganglion cells close 

 in ffont of the anus (anal ganglia), 

 and then the median nerve divides 

 in order to combine with the lateral 

 nerves on either side. In the male 

 the median nerve enlarges to nearly 

 the original number of fibres in 

 front of the anal ganglion, which 

 contains seven cells ; there is also 

 an anal ring embracing the terminal 

 gut, and there are two ganglion cells 

 in it on each side. In the dorsal 

 median ridge the dorsal median nerve 

 is alike in both sexes ; arising in 

 front with a single root from the 

 pharyngeal ring it gathers its fibres 

 from the lateral ganglia ; in the 

 anterior part of the body it con- 

 sists of thirteen to twenty fibres ; 

 in the posterior part of the body the fibres are reduced to four 



V.m.n.. 



A.- 



FIG. 185. Schematic representation 

 of the nervous system of a male, 

 Ascaris megalocephalus (after Brandes) 

 A., Anus; Ag. t Anal ganglion; C., 

 Commissures ; D.m.n., dorsal median 

 nerve ; Exp., excretory pore ; Pr., 

 pharyngeal ring ; Lg. , lateral ganglia ; 

 Ln., lateral nerve ; Sp., papilla of 

 sense ; V.m.n., ventral median nerve. 



