1 40 THE INVOL UNTAR Y NER VO US S YSTEM 



sympathetic cells ; a specially large mass known as Zucker- 

 kandl's body, which is found at the bifurcation of the ab- 

 dominal aorta, lasts till birth, closely connected with the inferior 

 mesenteric ganglia. As we pass downwards in the animal king- 

 dom, we find the chromaffine cells and the sympathetic cells 

 in close contact, even in the adult condition. In the Amphibia, 

 where the sympathetic ganglia are arranged much in the same 

 way as in the Mammalia, the chromaffine cells and the sympathetic 

 cells are mixed close together in every ganglion. So also in the 

 Elasmobranchs, the intimate juxta-position of the chromaffine cell 

 masses and the sympathetic ganglia has been long known, and 

 the origin of both from a common mass of cells was first de- 

 scribed by Balfour. 



In the Dipnoi, Teleostei, Ganoidei and Cyclostomata, Gia- 

 comini has shown that both systems lie in intimate connexion 

 with the cardinal and segmental veins, also that the relationship 

 between individual cells of the two systems is often extremely 

 close, a ganglion cell lying in close contact with a group of 

 chromaffine cells. 



The more primitive the animal the less conspicuous becomes 

 the sympathetic system. It is, according to Giacomini, present 

 in the Dipnoi, mainly in the form of two very fine lateral chains ; 

 but in the Teleostei, Ganoidei and Cyclostomata, it is not yet 

 aggregated into definite regular chains, but its cells are scattered 

 in irregular groups about the veins in company with the chro- 

 maffine cells, the component cells being still fairly numerous in 

 the Teleostei, less numerous in the Ganoidei, and extremely few 

 in the Cyclostomata. 



In the lowest group of vertebrates then the sympathetic cells 

 are so few that they cannot be described as forming a system ; 

 their place is taken by the masses of chromaffine cells, which form 

 a scattered but segmentally arranged system in the very position 

 occupied in the higher animals by the sympathetic system. 



In Petromyzon, groups of chromaffine cells are found in every 

 segment in exceedingly close contact with the cells of the posterior 

 root ganglion, where the segmental vein comes close against that 

 ganglion ; scattered groups of cells are found along the walls of 

 the segmental vein to its junction with the cardinal vein and 

 along the cardinal vein itself; also a few are found along the 

 peripheral branches of the segmental vein. My son finds these 



