THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
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and the large vessels are better developed, the cervical part of the cord lies in 
distinct relationship to the large arteries. It very plainly divides into two parts at 
the point of origin of the vertebral artery. One fibro-cellular cord accompanies that 
vessel, having arisen from the main cord at the point where the artery crosses it. 
Growth still proceeding, this part of the cord becomes more and more fibrous, until 
at length it assumes a condition characteristic of the adult vertebral plexus, and is 
gradually lost on the anterior part of the vessel. 
The other division, or main part of the sympathetic cord, accompanies the 
common and internal carotid arteries, at first only to the level of the mouth, where 
it forms a pointed extremity composed of cells, or a brushlike ending of cell processes. 
Growing larger along with the growth of the neck, this part of the cord becomes 
more and more constricted off from the part of the cord lying behind the level of 
the vertebral artery, by the formation of a fibrous commissure. A fibro-cellular mass 
is thus formed, which represents the “superior” cervical ganglion. The commissure 
connecting it posteriorly to the main cord is formed, partly by direct conversion of 
the cellular elements into fibres : partly by the passage into it of fibres derived from 
the anterior thoracic splanchnic branches. In this way the presence of a middle 
cervical ganglion may be explained, as representing a mass of cells derived from the 
elementary sympathetic cord which have been included among the fibres of the 
growing commissure. 
From the anterior end of the growing mass of the superior ganglion, a narrow 
bundle of fibres arises, which is closely applied to the internal carotid artery in its 
course beneath the auditory capsule. This bundle can be followed for a considerable 
distance, and is gradually lost upon the vessel, forming the carotid plexus. In fig. 17 
(Plate 29), the anterior end of the cervical ganglion {Si/.) is shown in a sagittal section 
of a Mouse embryo of 17-18 days, along with this fibrous bundle accompanying the 
internal carotid artery {Art.) beneath the Eustachian tube {East.). In this embryo 
the stream of fibres forming the vertebi’al plexus has much the same appearance as 
that shown in the figure. 
These parts—the vertebral and carotid plexuses, and superior cervical ganglion—I 
regard as pertaining to the collateral portion of the sympathetic system, for the 
following reasons :—(l), they are out-growths from the main cord (the ganglion, 
fundamentally a part of the original cord, undergoes extensive growth, and becomes 
soon constricted off from the main cord); and (2), no splanchnic branches enter this 
part of the cord. This agrees with Gaskell’s conclusion regarding the ganglion, 
which from morphological considerations he regards as belonging “ to the distal rather 
than to the proximal group of ganglia.” 
h. Caudal Termination. 
In the youngest embryos in which the sympathetic cord was found (Piat, 8 days, 
MDCCCXC. — B. 2 A 
