THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
1G9 
the fibres of the splanchnic branch can be seen coursing downwards and inwards. The 
splanchnic branch was not traced beyond the sympathetic cord. The latter was lost 
in the region of the neck in front, and in the region of the hind limb behind. 
In transverse sections of Rat embryos, at about 12 days, hardened in picric acid, 
and stained with borax-carmine, general development has advanced a stage (Plate 24, 
fig. 9), The cells of the intermediate cell mass have grouped themselves closely round 
the notochord (No.), in process of vertebra formation. The cells of the spinal cord 
{Sp.C.) are more distinctly separated into two gTOups, and the fibrous belt is thicker, 
especially near the origins of the spinal nerve-roots. The ventral root-fibres are seen 
to be in direct continuity with the cells of the spinal cord; the dorsal ganglion has 
its cells more regularly arranged in vertical ]’Ows. The divisions of the spinal nerves 
have increased in length, and are more distinctly differentiated. The splanchnic 
branch (Sj^l.) can, in transverse sections, be traced into the sympathetic cord, with 
which it is directly connected. In the thoracic region its fibres can be traced 
proximally through the spinal nerve and into continuity with both dorsal and ventral 
roots, Distally, its fibres can be traced into direct connexion with the cells of the 
sympathetic cord. In some cases the fibres are traced into the mass, and their cut 
ends appear in transverse sections. In the anterior part of the thorax, the fibres 
cannot be traced beyond the sympathetic cord. In the region of the neck, and in 
front of the origin of the vertebral artery, no splanchnic branch at all could be found 
joining the sympathetic. In some situations—especially in the region of the stomach 
—the splanchnic branch is divisible into two bundles of fine wavy nerves, one of 
which joins the sympathetic, and either communicates by its fibres with the cells, or 
is directed forwards or backwards in the cellular column ; the other bundle passes 
round the sympathetic cord, and can be followed in a ventral direction for some 
distance, in the interval between the aorta and cardinal vein. Here the wavy fibres 
are surrounded by large blastema cells, which lie pressed against their side ; among 
them, also, are seen other cells, directly continuous by their processes wuth the cells of 
the sympathetic cord. In other places, even more distinct cellular branches proceed 
peripherally from the latter, round the aorta, to form plexuses, collateral ganglia, &c. 
The sympathetic itself has much the same constitution as before, the noteworthy 
points in its structure being found at the ends. Posteriorly it narrows, and finally 
disappears at the point of bifurcation of the aorta. Traced forwards, it becomes 
thicker, attaining its greatest size near the origin of the vertebral artery. It here 
gives off a slender fibro-cellular cord which accompanies that vessel, and itself suddenly 
narrows and becomes decidedly fibrous. It soon swells out again, and resumes its 
cellular characters, and in this form is traceable to the level of the top of the pharynx, 
along the common and internal carotid arteries. Lying near the ganglion of the 
vagus, it becomes gi^adually narrower, and can be followed only as a thin fibrous cord 
along the carotid artery, as it courses beneath the auditory capsule, where it finally 
disappears. 
MDCCCXC.— H. 
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