DISTaiBUTIOV OF NON-MEDULLATED NERVE-FIBRES. 33 



As is well known, the larger nerve-trunks in the mesentery, 

 which consist of medullated fibres, and run a more or less 

 winding course, accompany the larger vessels from the mesen- 

 teric attachment towards the intestine. They give off com- 

 paratively few smaller branches, which are also sinuous, and, 

 further, are united together to form a network, the meshes of 

 which, from the small number of branches, are very wide. 



The branches of this plexus we will, for the sake of clear- 

 ness, call nerves of the first order. The medullated nerve- 

 fibres of which they consist lie generally close together ; 

 numerous oblong nuclei may be observed on their trunks, 

 as well as here and there isolated ganglion-cells, such as are 

 met with also elsewhere in the course of the sympathetic 

 nerves. 



From the smaller twigs of this plexus single nerve -fibres 

 are given off, which lose their medullated structure imme- 

 diately after they become separate. At certain spots these 

 fibres are obviously composed of fibrils exhibiting granular 

 enlargements ; the sheath in which they are contained is 

 comparatively broad, and contains numerous oblong nuclei ; 

 they divide (although not very frequently) into finer fibres, 

 and form by their union a plexus Avith wide areas, the 

 branches of which we will call nerves of the second order. 

 These nerves also are still provided with a nucleated sheath. 

 From this plexus there proceed, again, minute nerve-fibres, 

 and these are far more numerous than would be anticipated 

 from the inspection of ordinary preparations. 



These nerve -fibres, which we will call nerves of the third 

 order also run for the most part a winrling course, but some- 

 times bend abruptly at right angles to their original direction, 

 and finally give origin to very minute threads, the nerves of 

 the fourth order, which exhibit granular enlargements, and 

 are ultimately united into a network composed of almost 

 rectangular meshes. 



The point to be noticed in the nerve-fibres Avhich we have 

 called nerves of the third order, in contradistinction to those 

 described from the nictitating membrane, is that they exhibit, 

 at intervals, enlargements caused by oblong nuclei. These 

 enlargements are, nevertheless, in reality by no means so 

 numerous as they appear to be when viewed with a low 

 magnifying power (such as Hartnack's objective. No. 7), 

 since at many points where a nucleus appears to be inter- 

 calated in the course of the nerve-fibres, a higher magnifying 

 power (No. 9 or No. 10 immersion of Hartnackj shows the 

 appearance to be due to nothing more than a close apposition 

 of the fibres. 



VOL. XII. NEW SER. C 



