260 DR. P. CHALMERS MITCHELL ON THE 



large ganglion at entrauce of these to Meckel's tract, and I could make out only two 

 nerves forming the oi-igin of this ganglion. One of the three nerves frequently runs 

 independently to the duodenum, as in Otis (fig. 45) and in Andigena Bailloni ; but no 

 doubt there may have been a cross connection between this and the main intestinal 

 nerve, as certainly is the case in the Pigeon. In the duodenal-fold looj) usually at least 

 one distinct and large ganglion is present (fig. 45, v.ii.v.; fig. 72, d.g.). The main 

 intestinal nerve which supplies Meckel's tract starts in common with the duodenal 

 nerve as in Palamedea (fig. 1) and the Passeres (fig. 72). It courses round the mesen- 

 teric expanse of the tract, giving off a series of minute branches to the gut, and where the 

 tract passes into the rectum it frequently receives one of the entering nerves, or is in 

 connection with the ganglion at the entrance to the mesentery of Meckel's tract, and 

 then turns down parallel to the rectum, and comes again into connection with the 

 posterior portion of the Grenzstrang. On its course this main intestinal nerve may 

 follow Meckel's tract more or less closely. Thus in Palamedea {&g. 1), Pandiou (fig. 34), 

 Talegallus (fig. 38), it foi'ms a loop corresponding to, but not absolutely identical with, 

 the course of the tract. In Falco melanogenys (fig. 32) it follows the general course of the 

 tract much more closely, having a loop following the supra-duodenal loop. In Crax 

 Danbentoni (fig. 39) it has a curious elongated and narrow distal portion, which corre- 

 sponds exactly with the typical apocentricity of these birds, in which the tract itself is 

 enlarged on its distal portion. This intestinal nerve in many cases — Struthious birds, 

 Palamedea (fig. 1), Talegallus (fig. 38), Crad- (fig. 39), other Galli, Falco (fig. 32), 

 Pandion (fig. 34) — presents a very large numljer of small ganglia evenly distributed along 

 its course. In other birds, e. g., Otis (fig. 45), Columba, Andigena, and Corviis (fig. 72), 

 there are a smaller number of much larger ganglia. As to the systematic value of these 

 differences, it is impossible to say anything definite on so small a range of information ; 

 but the sixbject is very promising. It appears as if the chain with many ganglia were 

 more primitive than the nerve with a limited number of large ganglia ; and it is certainly 

 the case that such birds as Otis, Columba, the Toucan, and the Passeres, Avhere the 

 number of ganglia is very small, are more specialized forms than those with multi- 

 ganglionated chains. It has to be remembered, however, that although the nervous 

 systems of many invertebrates with a small number of large ganglia appear to have been 

 produced by the concentration of multi-ganglionated chains, we have no right to extend 

 such a princi])le to other forms. The origin of this intestinal nerve in birds requires to 

 be worked out, and in this connection the observations of Andersson (i), who found that 

 in the Urodele Amj)hibia the main symjiathetic chain was subdivided into a Grenzstrang 

 and a Collateralstrang , are worthy of attention. 



It is to be noticed that this intestinal nervous system lies, as the blood-vessels lie, 

 between the two layers of the mesentery, and therefore outside the coelom. In larger 

 forms, such as the Emu, it is often comparatively easy to strip otf one layer with its 

 load of fat from the mesenterial expanse supporting Meckel's tract, upon which the 

 ganglionated nerve-chain come plainly into view\ However, just as it occasionally 

 happens that some of the blood-vessels pierce the mesentery and form *' bridging" vessels 

 traversing the portion of coelom between two loops which happen to lie in contact, so in 



