vi SYMPATHETIC SYSTEM 363 



The second variety are the so-called fibres of IJemak, which 

 have no niyelin sheath and present a grey appearance. They are 

 derived from the cells of the sympathetic ganglia, and connect 

 these with the peripheral organs. 



The general rule that the white (myelinated) fibres of the 

 sympathetic system descend from the spinal cord (efferent paths) 

 or ascend to the cord (afferent paths) and thus belong entirely to 

 the cerebrospinal axis, while the grey (non-medullated) fibres 

 spring from the sympathetic ganglia and belong to the sympathetic 

 system in the narrower sense, is, according to Langley, liable to 

 exceptions. For example, he says that the nerves of the sym- 

 pathetic ganglia which innervate the muscles of the feathers in 

 birds are all myelinated. 



II. The peripheral organs supplied by the fibres of the sym- 

 pathetic have already been discussed in the preceding volumes, but 

 may here be recapitulated : 



(a) The sphincter of the iris and pupil, contraction of which 

 diminishes the size of the pupil. 



(&) The ciliary muscles, contraction of which relaxes the zonule 

 of Zinu and accommodates the eye for near objects. 



Both these muscles are innervated, as are most of the striated 

 muscles of the eye, by the third cranial nerve ; but the fibres 

 destined for these unstriated muscles are, unlike those for the 

 other muscles, interrupted in a peripheral ganglion, the ciliary 

 ganglion, like all sympathetic fibres ; accordingly, they must be 

 included in the sympathetic system. 



(c) The dilatator pupillae, the functional antagonist of the 

 sphincter of the pupil, contraction of which widens the pupil. 



(rf) The plain muscle fibres found in the orbital tissues, 

 Tenon's capsule, and the eyelids, which enlarge the palpebral 

 fissure, protrude the eyeball and retract the eyelids. In some 

 animals there are also muscle fibres in the nictitating membrane 

 in the internal angle of the orbit, contraction of which retracts 

 this membrane. 



(f) The musculature of the blood-vessels of the eye, which 

 when contracted constricts the blood-vessels of the conjunctiva, 

 iris, etc. The lachrymal glands also receive fibres from the 

 sympathetic, which on stimulation cause a secretion of tears. 



All these nerve -fibres spring from the superior cervical 

 ganglion. We shall later discuss their connections with the 

 spinal cord. 



Next to the sympathetic fibres that supply the eye come those 

 which are distributed to the surface of the body to innervate the 

 plain muscle fibres of the various organs of the skin. These 

 are : 



(a) The muscle fibres of the cutaneous vessels, contraction of 

 which constricts the vascular lumen and diminishes the amount 



