178 ME. F. J. COLE ON THE STEUCTUliE AND MOKPHOLOGY OF 



O. MoEPnoLOGiCAL Value of the Lateralis Nerve of Petromtzon 



Whilst working at the accessory lateral system of Teleosts, my attention became 

 directed to the " lateralis " nerve of Petromyzon, and it occurred to me, whilst reading 

 Ahlborn's second paper on the Lamprey, that this nerve was not, as is usually supposed, 

 a lateral line nerve, but really belonged to the accessory lateral system. The fact that a 

 definite lateral or body canal is unrepresented in Petromyzon, or at the best only 

 represented by a few scattered sense organs, shows that the nerve, whatever it may 

 be, is certainly not a lateral Hue nerve, which confirmed my former impression and 

 necessitated fvu'ther enquiry into the question. 



As far back as 1849, Stannius ( 1 99, pp. 95 and 96) had maintained tliat the lateral nerve 

 of the Lamprey was not a lateral line but a general cutaneous nerve, but did not compare 

 it with his " dorsale Schedelhohlenast." Ahlborn (1884, 2) described an anastomosis 

 between the " lateralis " and the facial, whicli has been compared both with the lateral 

 line anastomosis of Pi^otopterus and also with Jacobson's anastomosis. Erom wliat has 

 l)receded it is impossible that it can represent either of tliese nerves. If, on the other 

 hand, my suggestion be correct, it is easy to see that this anastomosis represents the 

 anterior or trigemino-facial root of the accessory lateral system. 



The short but important paper by E-ansom & Thompson (1886, 165) conclusively 

 shows (1) that the " lateralis " of Petromyzon is a somatic sensory nerve, and (2) that it 

 is connected loith the dorsal roots of the spinal nerves. They further state that it hos 

 no ganglion, and consider it to represent a commissural systetn between the dorsal roots 

 of the spinal nerves. Dohrn (1888, 62) has endeavoured to show that there is no 

 connection between the spinal and '• lateralis " nerves, and Beard, believing the latter to 

 be a true lateral line nerve, also discredits the assumption. He says (1888, 2 1 , p. 215) : 

 " We know nothing of su^ch connection of spinal nerves with the sense organs of the 

 lateral line, either now or in the past, and any opinion one may express in favour of 

 such a view is only an assumption." This is quite true, but does not, as wc have seen, 

 apply to Petromyzon. Dohrn's denial of Ransom & Thomj)son's statements has not 

 been confirmed by svibsequent investigation, which has indeed completely established 

 the connection between the " lateralis " and spinal nerves of Petromyzon. 



Eisi"' (1887, 65), who believed the lateral sense organs to be sometimes innervated by 

 spinal nerves, naturally considered the " lateralis " of Petromyzon a lateral line nerve, 

 whicli conclusion was also taken for granted by Julin (1887, 109). This author, whose 

 conclusions were reprinted in his larger work published in the same year (112), confirms 

 Eansom & Thompson's statements as to the connection of the " lateralis " \\\W\ the spinal 

 nerves *, but states further that it is connected wdth the ventral roots also. His admission 

 at the outset of Langerhans' contention that the Lamprey's " lateralis " is a true 

 lateral line nerve is, omitting that it is a j)ure assumption, unfortunate, since he is 

 thereby induced to formulate an elaborate theory as to how the connection came about. 



* Juliu states in a footnote at the end of his paper that he has discovered connections between the lateralis 

 lateral line nerve and the dorsal roots of the spinal nerves in cmbrj'os of Scyllrimi and Splnax. This connection, to 

 my mind most im])robablc, has never been confirmed. 



