THE SYMPATHETIC NERVOUS SYSTEM $29 
CoccYGEAL NERVES 
The coccygeal nerves (Nn. coccygei) commonly number five pairs. Their 
dorsal and ventral branches anastomose to form respectively two trunks on either 
side, which extend to the tip of the tail and supply its muscular and cutaneous 
nerves. The dorsal trunk runs with the dorso-lateral artery between the sacro- 
coceygeus dorsalis and intertransversales muscles (Fig. 579). The ventral trunk 
accompanies the ventro-lateral artery below the intertransversales. 
THE SYMPATHETIC NERVOUS SYSTEM! 
The sympathetic nervous system (Systema nervorum sympathicum) is that 
part of the nervous system which serves (1) to transmit stimuli to the heart muscle, 
unstriped muscle, and glands; (2) to conduct impulses from the viscera to the cere- 
brospinal system. 
Many of the fibers are derived from the cerebrospinal system and are rearranged and dis- 
tributed in the sympathetic system. Numerous sympathetic fibers are contributed to the cerebro- 
spinal nerves, through which they are distributed to the unstriped muscular tissue and glands, 
as vasomotor, pilomotor, and secretory nerves. The sympathetic, like the cerebrospinal system, 
consists of neurones, each of which comprises the cell-body, an axone, and numerous branched 
dendrites. The cell-bodies are aggregated into ganglia, some of which are large and more or less 
constant in position and form, while others are microscopic and are scattered in an irregular man- 
ner through the peripheral part of the system. Simple visceral reflexes may be mediated by sym- 
pathetic neurones alone. 
In descriptive anatomy the sympathetic system is regarded as consisting of 
(1) a chain of ganglia extending along each side of the vertebral column and con- 
nected by association fibers to form the sympathetic trunk; (2) central branches 
to and from the cerebrospinal nerves; (3) peripheral branches, which form plexuses 
(Plexus sympathici) with each other and the cerebrospinal nerves; (4) the peripheral 
ganglia which are interposed in the plexuses (Ganglia plexuum sympathicorum). 
The sympathetic trunk (Truncus sympathicus) extends on either side from 
the base of the cranium to the tail. In it are interposed, at intervals of varying 
regularity, the ganglia of the sympathetic trunk (Ganglia trunci sympathici). 
These are connected with the cerebrospinal nerves by central branches, the rami 
communicantes. 
Two kinds of rami communicantes occur. Of these, one type consists largely of medullated 
fibers derived from the spinal nerves and ganglia; they have therefore a white appearance, and 
are termed white rami. They contain both efferent and afferent fibers. The efferent splanch- 
nic fibers are derived from the ventral roots of the spinal nerves and terminate in great part 
about the cells of the nearest sympathetic ganglion; others end in more distant or in peripheral 
ganglia. The afferent splanchnic fibers are chiefly peripheral processes of the cells of the spinal 
ganglia, but some are sympathetic fibers which enter the spinal nerve-trunk and terminate about 
cells of the spinal ganglion. The gray rami consist mainly of non-medullated fibers derived from 
the sympathetic ganglia directly or through the trunk, which proceed centrally to the spinal 
nerves and are distributed along the somatic divisions of the latter to unstriped muscle and glands 
as vasomotor, pilomotor, and secretory fibers. Some go to the membranes of the spinal cord, 
and a few terminate about cells of the spinal ganglia as sensory sympathetic fibers. 
Similar but more complex and irregular communications which exist between 
the sympathetic system and the cranial nerves—with the exception of the first 
and second—have been referred to in the accounts of the nerves. 
It is convenient for descriptive purposes to divide the sympathetic system into 
cephalic, cervical, thoracic, abdominal, and pelvic parts. 
1. The cephalic part (Pars cephalica systematis sympathici) comprises the 
otic, sphenopalatine, and ciliary ganglia, which may be regarded as homologues of 
the ganglia of the trunk of other regions. It also includes three plexuses formed by 
_. 1 The special statements of this description refer to the system of the horse. A few important 
differential features will be mentioned in the account of the nervous system of the other animals. 
