OP THE FORMATION OF THE SPINAL MARROW AND BRAIlSf. 53 



however, any crossing of fibres being 1 manifest at the point where the junction of both 

 nerves take place. 



M. Treviranus has, in a great measure, confirmed these observations, on the male 

 simia avgiila. The nerves and brain were left during some months in alcohol, and af- 

 terwards kept some time in caustic potash to soften them. Having thus prepared 

 them, he submitted them to a careful dissection, when he made out, with the aid of 

 a microscope, that the external fibres of the upper side of each were continued from 

 their cerebral extremity to that in the eye, without uniting 1 themselves to those of the 

 other side ; whilst, on the contrary, the internal and inferior fibres of one nerve went 

 to the other side, and united with the fibres of the opposite nerve. It was difficult to 

 determine whether any of the fibres actually passed from one side to the other. He 

 thought, however, that sume of the fibres di'd so. The internal fibres, thus interlacing 

 together, were evidently more numerous than the external fibres which ran to the eye 

 without uniting with those of the opposite nerve Journal Complementaire, Oct. 1823. 



On the Motions of the Eye. 



Mr. Charles Bell has lately examined* the motions of the eye, in illustration of the 

 uses of the muscles of the orbit; and has shown, in the first place, that there are mo- 

 tions performed by this organ not hitherto noticed. Every time the eyelids descend to 

 cover the transparent part of the eye, the eyeball ascends, or suffers a revolving mo- 

 tion. If this were not the case, the surface of the eye would not be moistened, nor 

 freed from offensive particles. He has proved, in the next place, that during sleep the 

 eyeball is turned up, and the cornea lodges secure and moistened by the tears, under 

 cover of the upper eyelid. He considers that these motions are rapid and insensible, 

 and that they are provided for the safe-guard of the eye. The other motions are vo 

 luntary, and for the purpose of directing the eye to objects. 



Mr/Bell next examined the actions of the muscles of the eyeball, and distinguished 

 them, as usual, into the straight and oblique muscles. It has been supposed, hitherto, 

 that both these classes of muscles were voluntary ; some describing the oblique as coad- 

 jutors of the recti muscles, and others as opponents to the recti ; but Mr. Bel! has 

 viewed the oblique as provided for the insensible motions of the eyeball, and the recti 

 for those motions w T hich are directed by the will, and of which we are conscious. 



Mr. B. has also proceeded to show, that, the consciousness of the action of the recti 

 muscles gives us the conception of the place or relation of objects ; and has endea- 

 voured to prove, by observation and experiment, that the actions of the straight mus- 

 cles are inseparably connected with the activity of the retina; that is, with the enjoy- 

 ment of vision : but that the moment the vision is unexercised, the eyeball is given 



May, 1823, 



Of the Formation of the Spinal Marrow and Brain. 

 Note DD. 



I. Of the Formation of the Spinal Marroio. The researches of M. Tiedemann, the 

 Wenzels, Doellinger, Cams, and Desmouiins, have furnished us with much interesting 

 information on this subject. The soft and gelatinous state of the embryo at the ear- 

 liest periods of its existence, the rapidity of its metamorphoses, and the difficulty of de- 

 monstrating, owing to its colour and consistency, that part of the nervous system, which 

 from the circumstance of its supplying those parts of the embryo that are first formed, 

 as well as from other considerations, we are led to consider as the first which assumes 

 an organized appearance, combine to render the exact origin of the spinal marrow diffi- 

 cult, if not impossible of demonstration. Reasoning from the condition of the nervous 

 Astern throughout the scale of the animal kingdom, from the manner in which the dif- 



