622 



SKELETON. 



3. A very fine filament from the sixth nerve 

 to the ciliary or lenticular ganglion has been 

 described by several authors. 



Subsequently to the cavernous sinus, the 

 course of the nerve is but short. Arriving at 

 the posterior extremity, or apex of the orbit, 

 the nerve lying to the outer side of that part 

 of the third which supplies the inferior rectus 

 and oblique muscles, runs slightly upwards, 

 and turning outwards, continues for a very 

 short distance along the inner surface of the 

 external rectus. It finally breaks up into 

 numerous minute filaments, which enter the 

 ocular surface of this muscle to be distributed 

 to it. 



Physiology of the sixth nerve. The function 

 of the nerve is, perhaps, sufficiently indicated 

 by the preceding details. Since anatomy shows 

 that its terminal distribution is exclusively to 

 a muscular surface, we should on this ground 

 alone be tolerably entitled to predicate its 

 motor function. 



The little that is known of its comparative 

 anatomy confirms the inference. In all the 

 higher vertebrata it is distributed to the ex- 

 ternal rectus. In some, however, it experi- 

 ences an enlargement, and a further distribu- 

 tion. The muscle which sweeps the broad 

 nictitating membrane over the bird's eye, and 

 the funnel-shaped, or choanoid muscle which 

 surrounds the optic nerve and eyeball of many 

 mammalia, are both supplied from this nerve. 



So also one or two cases are recorded, in 

 which an injury of this nerve from disease in 

 the neighbourhood has produced paralysis of 

 the external rectus, and an inward squint. 

 While, vice versa, the experiment of galvanising 

 the nerve has been accompanied by violent 

 contractions of the muscle, and an external 

 strabismus. 



The insensibility of the nerve is, perhaps, 

 less certain than might at first appear, though 

 Longet* distinctly states that pinching the 

 nerve at its origin is unattended by signs of 

 pain. The branch of junction with the oph- 

 thalmic nerve seems to be, from its direction 

 and appearances, much more like a filament 

 from the sensitive to the motor nerve, than 

 from the latter to the former. If this be the 

 case, they would seem to be somewhat ana- 

 logous to the junction of the numerous 

 branches of the fifth with the portio dura on 

 the face. And in the absence of direct ex- 

 periment upon the nerve beyond the seat of 

 this union, one might conjecture it as possible, 

 that the sixth nerve was possessed of a slight 

 sensibility similar to that of this portion of the 

 seventh. Concerning the import of the junc- 

 tion with the sympathetic, little can here be 

 said ; for although, as compared with the size 

 of the communicating nerves, this union is 

 larger than most others, yet there does not 

 seem any sufficient reason for supposing other 

 differences. 



The distribution of a branch from the sixth 

 to the ciliary ganglion has been thought by 

 Longet and others to explain the persistence 



* Sur le Systeme Nerveux. 



of movements of the iris after paralysis of the 

 third nerve. But besides that the constant 

 existence of this filament seems hardly veri- 

 fied ; perhaps the interposition of a ganglion 

 between the paralysed nerve and the ciliary 

 filaments might alone be thought a sufficient 

 explanation of the inconstancy or imperfec- 

 tion of the result, without requiring the ex- 

 istence of another and an uninjured channel 

 as the cause. 



Bibliography. See " NERVE." 



(William Brinton.') 



SKELETON. The name skeleton, <r x e- 

 Xcrov, formed from <7%6\\a>, to dry, is, in 

 anatomy, ordinarily applied to denote that 

 assemblage and arrangement of all the osseous 

 pieces of an animal framework in such con- 

 nection and relationary order as the hand of 

 nature has disposed them for fitting operation 

 in the living body. 



The less the name skeleton impresses the 

 mind with the configuration of any particular 

 form of the osseous machines, the better is 

 it fitted as an abstract general title, under 

 which to give a comparative survey of all 

 figures of the osseous system, whatever be 

 their special characteristics ; and this abstract 

 survey being my present purpose, I find that 

 the name skeleton, devoid as it is of any direct 

 and inconvertible meaning, conveniently ex- 

 tends itself over all varieties of the osseous 

 fabrics of the four higher classes of animals ; 

 from the mutual comparison of which I shall 

 strive to elicit the law which creates them in 

 the character of a unity in variety *, a condi- 

 tion of form by which the many species gather 

 themselves together naturally into a circle and 

 point to some unknown oneness of character 

 which enchains them the one to the other. 



This law of unity in variety is still uninter- 

 preted ; and though it formed the moving 

 theme of the great Grecian naturalist f three 

 thousand years back, and afterwards lay in 

 cold obstruction till resumed in later times 

 by Leibnitz, Newton, Buffon, Cuvier, Geoffroy 

 St. Hilaire, Oken, Goethe, Carus, Owen J, 



* Leibnitz makes use of this phrase as being the 

 general expression of his ideas of that condition of 

 development manifested throughout the animal 

 kingdom, namely the condition of an all-encom- 

 passing structural analogy which relates organised 

 beings more or less closely to one another. His 

 " loi de continuite " is founded likewise upon the 

 same general fact. He defines the universe as 

 " 1'unite dans la variete," and of the animal king- 

 dom he writes, " tout va par degres dans la nature, 

 et rien par saut." See CEuvres Philosophiques de 

 M. de Leibnits, liv. p. 440. 



f Aristotle, the great founder of generalisation 

 in the physical sciences, was strongly impressed 

 with the common resemblances or analogies of ani- 

 mals, and expresses the fact as follows : " But 

 some animals neither have parts specifically the 

 same, nor the same according to excess and defect, 

 but according to analogy." History of Animals, 

 book i. p. 4. trans, by* Taylor. 



J The late work of the learned Hunterian profes- 

 sor, entitled " Homologies of the Vertebrate Skele- 

 ton," contains, in addition to his own especial views, 

 a complete account of all that has been written upon 

 the subject of skeletal analogies by the leading com- 



