APPENDIX. 545 



which plexus two, three, or more chords pass into the substance of the 

 arachnoid. The shape of these plexuses is either square or triangular, 

 according to the number of branches which join them, and the number 

 they give off. Besides consisting of interlacing fibres, they also contain 

 corpuscular matter. 



" The arachnoidal extremity of some of the chords connecting the 

 vessels with the arachnoid of the cauda equina expands, close to the 

 membrane, into a large oblong and rather oval bulb, the axis of which 

 is occupied by a continuation of the chord, extremely convoluted and 

 bent upon itself; whilst, inferiorly, its fibres are blended with those of 

 the membrane. 



" The chords which pass from the vessels of the pia mater, at the 

 upper portion of the brain, to the arachnoid, terminate in the latter by 

 fibres having a stellate arrangement. There are also some large triangular 

 plexuses like those at the base of the brain, from which branches descend 

 between the convolutions to the vessels within the sulci. 



" In the lower animals, as in the sheep, in which the cerebral convolu- 

 tions are small, the stellate fibres are the best seen. They can even be 

 distinguished by the naked eye, appearing like minute opaque points. At 

 their centre, the fibres of which they are composed, seem to be blended 

 into an irregular confused mass, from which other fibres radiate, and lose 

 themselves in the cerebral surface of the arachnoid. Some fibres go from 

 one stellate body to another, and others can be traced into the coats of 

 the vessels : these latter are by no means numerous. Branches also de- 

 scend (still having somewhat the stellate disposition) between the con- 

 volutions to the deep-seated vessels; these filaments are much more 

 numerous upon some vessels than upon others, and they do not appear to 

 extend so far as the capillaries, no fibres of any kind being visible upon 

 this system of vessels. 



" It appears, from what has been stated, that the disposition of the 

 ramifying filaments of the arachnoidal chords, and the form and size of 

 the gangliform plexuses connected with them, bear some proportion to 

 the number and size of the vessels in their vicinity. Hence, about the 

 base of the brain, where the branches of the arteries are large, the 

 plexuses are also large, and of an irregular shape, whilst on its upper 

 surface, where the vessels are comparatively small, and more equal in 

 size, and have a more uniform distribution, the plexuses also are smaller, 

 more numerous, and more regular in their shape and volume. 



" Besides the plexuses situated in the course of .the chords of the 

 arachnoid, there are others which are more intimately connected with its 

 cerebral surface, and which, in some situations, appear to compose the 

 entire thickness of the membrane. 



" In these plexuses, the filaments interlace very much in the same 

 manner as the nerves do in the plexuses of the cerebro-spinal and sym- 

 pathetic systems. A chord, for instance, when traced into one of them, 

 will be observed to break up into its component filaments, the adjoining 



u u 4 



