CEPHALIC NERVES. THIRD, FOURTH, AND SIXTH. 685 



whilst by another small root it is connected with the cavernous plexus of the 

 Sympathetic system : thus presenting a sort of miniature representation of the 

 entire series of Sympathetic ganglia, and of their connections with the Cerebro- 

 spinal system. 1 



715. The Third, Fourth, and Sixth pairs, together make up the apparatus 

 of motor nerves, by which the muscles of the Orbit are called into action. The 

 3d pair supplies the greater number of the muscles ; the 4th being confined to 

 the Superior Oblique, and the 6th to the External Rectus. Of these nerves, 

 the 3d pair is the only one which exhibits any appearance of sensibility, when 

 its trunk is irritated; but this sensibility is not nearly so great as that of the 

 5th pair; and as there is no reason to believe that it is really possessed by the 

 3d in virtue of its direct connection with the nervous centres, it is probably 

 imparted by the anastomosis of that nerve with the 5th some filaments of 

 which may pass backwards as well as forwards, so as to confer sensibility on the 

 trunk of the 3d, above as well as beyond their point of entrance. The peculiar 

 mode in which those motor nerves ordinarily excite the muscles to action, under 

 the guidance of the visual sense, will be considered in the next Section. Although 

 commonly ranked as cephalic nerves, they have no direct connection with the 

 Cerebrum; their real origin being from the upper part of the Spinal Axis ( 712). 

 The roots of the third pair may be traced into direct connection with the Corpora 

 Quadrigemina ; a fact of considerable physiological importance, as will hereafter 

 appear. The chief actions of a purely reflex nature, to which this group of 

 nerves ordinarily ministers, are the government of the diameter of the pupil, 

 which is accomplished through the Third pair ; and the rolling of the eyeball 

 beneath the upper lid during sleep, as well as in the efforts of sneezing, cough- 

 ing, &c. But irregular movements of the eyeballs, which must be referred to 

 the same group, are continually seen to accompany various other forms of con- 

 vulsive action. 



716. The Portio Dura of the Seventh pair, or facial nerve, has been con- 

 sidered, since the first researches of Sir C. Bell, to be a nerve of motion only ; 

 but some physiologists have maintained, that it both possesses sensory endow- 

 ments, and" arises by a double root. According to Yalentin, however, who 



1 The functions of this ganglion have been made the subject of particular investiga- 

 tion by Dr. C. Radclyffe Hall ("Edinb. Med. and Surg. Journal," 1846-48), whose most 

 important results are as follows : 



1. The size of the ciliary ganglion is always in direct proportion to the activity of the 

 iris, which in turn always bears a direct relation to the strength and acuteness of vision, 

 and to the nocturnal habits of the animal, and implies a proportionate development of the 

 internal vascular apparatus of the eye. 



2. The ganglion is always more intimately connected with the 3d pair than with any 

 other, the size of the short root being always in direct relation to that of the ganglion ; 

 and the ganglion being sometimes a mere swelling on the trunk of the nerve. 



3. The fibres derived from the 5th pair do not terminate in the ganglion, but pass on- 

 wards through it to the ciliary plexus. 



4. In the Rabbit, the iris receives fibres from the 6th pair, which do not pass through 

 the ganglion ; and it is through this that the contraction of the pupil is produced in that 

 animal by irritation of the 5th pair, which will not produce any efiect upon the pupil of 

 the Dog, Cat, or Pigeon, so long as it does not affect the brain to the extent of producing 

 vertigo, nor affect the visual sense in any other way. 



5. Irritation of the 5th nerve does not in any animal affect the action of the iris after 

 the division of the cerebral connections of all the other ocular nerves ; so that its influ- 

 ence over the movements of the iris must be reflected through the encephalic centres, not 

 through the ophthalmic ganglion. 



6. The function of the ganglionic centre itself, as a part of the Sympathetic system, 

 is to bring the " organic actions" of the eyeball, especially its supply of bloqd, into har- 

 mony with its functional activity ; this harmony being produced by the passage of the 

 cerebro-spinal nerves through the ganglion, which excites the synergetic action of its own 

 vesicles and nerve-fibres. 



