THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
171 
lumbar regions. Here the splanchnic branch [spl., Plate 2G, fig. 13) is distinctly 
formed of two parts. Each consists of fine nerve-fibres, among and around which are 
long fusiform mesodermal cells, forming a sheath for them. Reaching their extremities, 
the fibres of each portion separate into hiindles of fine, wavy fibrils. These fibrils, 
in the case of that portion (a) which joins the sympathetic {Sy.), join the cells or 
separate out among them. The portion (/3) which passes beyond tlie sympathetic 
cord, is accompanied by a cellular branch (y) derived from the latter, and the two 
proceed in a venti’al direction round the aorta. Around the coeliac axis, the cellular 
branches from both sides, coursing round the aorta, are now seen to unite together to 
complete the solar plexus. Both splanchnic branch and cellular branch from the 
sympathetic are larger and longer than before. 
In transverse sections of a Human embryo about the end of the first month, 
hardened in spirit and stained with aniline blue-black, the sympathetic cord has very 
much the characters just described. The cord itself is large and uniform in width, 
widening out anteriorly to form the inferior cervical ganglion ; beyond this it narrows, 
encloses the subclavian artery, and forms a fibrous cord; this again becomes cellular, 
and widens out into the “ superior ” cervical ganglion. No splanchnic branches join 
the cord in front of the level of the inferior cervical ganglion. In the thorax 
(Plate 28, fig. 16) the splanchnic branches are seen {spl.) arising from both roots 
of the spinal nerve {I.D.), and, as in the figure, terminating wholly in the sym¬ 
pathetic cord {sy.). Sometimes a small portion of a splanchnic branch can be traced 
round the ventral side of the cord, accompanied by a cellular branch from it. In the 
hinder thoracic region, a small part only of the splanchnic branch joins the cord, the 
greater part, along with cellular outgrowths from the sympathetic, passing onwards 
to form the solar plexus and semi-lunar ganglia, which are seen in process of 
formation on the ventral aspect of the aorta. A similar fibro-cellular bundle passes 
to join the suprarenal body. In the lumbar region the splanchnic branch can be 
seen for a considerable distance almost entirely unconnected with the sympathetic 
cord, and separated by an interval from it. The cord gradually narrows as it is 
followed backwards, and, becoming attenuated, disappears at the point of bifurcation 
of the aorta. 
In Rat embryos at about 15 days, stained with borax-carmine or aniline blue- 
black, there is not much further change. The sympathetic cord has much the 
same extent as before (Plate 27, figs. 14 and 14 a, Ny.); is uniform in width for the 
most part (except that it is considerably thickened in the situation where the fibro- 
cellular bundle arises to join the solar plexus, semilunar ganglion, and suprarenal 
body); and consists of cells which, especially anteriorly, are long, narrow, and fusiform, 
with deeply stained round or oval nuclei, and terminal filiform processes. In some 
cases the cells seem almost converted into fibres. Near the posterior end the cells are 
wider and shorter. The cell processes are seen to join the splanchnic branches 
directly. Careful examination of the anterior nerves strengthens my confidence that 
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