ORGANS OF THE BODY. 



577 



Of the veins, two are specially striking in the neighbourhood of the 

 central canal (Clarke, LenhosseK). 



Some years ago the arrangement of the capillary network of the cord 

 was very minutely studied by Gull. The widest meshes were found by 

 him in the anterior columns, the finest in the lateral, while those of the 

 posterior columns lay between the two. But the densest capillary inter- 

 lacements of all are to be met with in the grey matter in those situations 

 where many ganglion cells are collected together. Finally, the restiform 

 bodies are remarkable for having meshes of capillaries as small as those 

 of the grey substance. 



It has been already mentioned ( 207) that throughout the whole spina] 

 cord and brain, all the blood-vessels, including arteries, veins, and capil- 

 laries, are invested with a loose sheath of connective-tissue. A watery fluid 

 found within tho latter has been regarded by some as lymph. The existence, 

 however, of this system of perivascular canals of His is not yet firmly 

 established, and has lately been the subject of very earnest controversy. 



293. 



Having now discussed the connective-tissue groundwork of the cord, 

 let us turn to its nervous elements. 



The white substance, as has been already remarked, consists entirely of 

 fibres. These present all the characters of central structures (fig. 556,/, g, h), 

 i.e., they are not supplied with the same primitive sheath as the peripheral 

 tubes, so that in many cases we are only able to obtain them in broken 

 fragments. In the finer specimens further there seems to be a tendency to 

 varicosity ( 176), and we may easily recognise their axis cylinder. Their 

 diameter may be roughly estimated 

 at from 0'0029 to O0090 mm., show- 

 ing, that besides very fine elements, 

 broader ones also exist. It appears 

 beyond doubt, further, that these central 

 fibres divide, although we are confined to 

 conjecture at present as to the frequency 

 of the occurrence. 



Passing on to the arrangement of the 

 nerve fibres in the white columns of the 

 cord (fig. 557), we have to discriminate 

 between bundles which hold a longitu- 

 dinal, a horizontal, and an oblique course. 

 The greater proportion of fibres belong to 

 the first of these classes (I, tn, n], and are 

 often unmixed with bundles having any 

 other direction. Their course in the 

 peripheral portions of the cord is regularly 

 parallel, while in the vicinity of the grey 

 matter they generally may be observed 

 to interlace, and to be collected in small 

 fasciculi. 



Further, and in this we have probably 

 an important physiological fact, certain 

 regular differences in the diameter of those nerve fibres of the white 

 columns are manifest. 



In the first place, the more internal, lying close to the grey matter, 



Fig. 556. Different kinds of nerve fibre,*, 

 f, g, h, central ; the fibre g, as axis cylin- 

 der, is continuous above, with the pro- 

 cess of a ganglion cell. 



