782 



ORBIT. 



tremity of the cut nerve produced contraction 

 of the pupil, although under the circumstances 

 the animal could not have been conscious of 

 such irritation. 



Radiated or sympathetic sensations. 



The optic nerve participates in a class of ob- 

 scure sensations to which a brief allusion may 

 be here permitted ; these are called radiated or 

 sympathetic sensations; they occur occasion- 

 ally in health, though they are more frequently 

 symptomatic of disease or irritation elsewhere 

 situated, and as they are likewise manifested 

 by other nerves it would be erroneous to con- 

 sider them peculiar to the optic. The pheno- 

 mena to which allusion is made are for the 

 most part transitory affections of sensitive 

 nerves which do not seem to depend on any 

 direct impression made upon the nerve af- 

 fected, but rather to be produced by causes 

 which act on other (generally distant) parts of 

 the system. 



The following will serve as examples of the 

 affections under consideration. 



A discordant sound, such as that produced 

 by setting a saw or scratching glass, gives rise 

 to shuddering, or a sensation as if water were 

 dropping over the surface. Tickling the soles 

 of the feet occasions general sensations of the 

 most disagreeable nature; and the impression 

 of a strong light on the eye is often followed 

 by a sense of irritation in the nose, with violent 

 sneezing. 



Similar affections of the optic nerve will 

 readily occur to the reader's recollection ; thus 

 various forms and degrees of temporary insen- 

 sibility or excitement of the retina, which are 

 known to depend on gastric disturbance, belong 

 to this category, and many other such instances 

 might be adduced. 



Facts, such as the foregoing, have long been 

 familiar to physiologists; but to account for 

 them seems still to be a matter of difficulty. 

 The supposition that the connections of the 

 sympathetic with the nerves affected explain 

 the problem is far from satisfactory ; the 

 most plausible theory is that which supposes 

 the primary irritation to be propagated in 

 a centripetal direction along the nerves of the 

 part to the cerebro-spinal centres, and thence 

 reflected upon the roots of those nerves in 

 which the sensations are developed, in some- 

 what the same way that excito-motory impres- 

 sions on nerves come to produce reflex motions; 

 the difference in the two cases amounting to 

 this, that in the one the primary impression 

 reacts upon motor nerves, giving rise to reflex 

 motions, in the other on sensitive nerves causing 

 thereby reflex sensations. 



Though this may be the true explanation, it 

 is nevertheless not perfectly satisfactory, for it 

 does not shew why the reflex irritation should 

 be prone to fall on one sensitive nerve in pre- 

 ference to another, yet the optic is known in 

 such cases to suffer more frequently than the 

 auditory or olfactory. 



BIBLIOGRAPHY. The following books may be 

 referred to in addition to the several sytematic 

 treatises on Physiology and Descriptive Anatomy. 

 Sir J. Newton, His optics, query 15, London, 1718. 



John Swammerdam, Book of nature, translated by 

 Thomas Flloyd, 1758. Sam. Tliom. Sasmmerinff, lie. 

 basi encephali et originibus nervorum, 1778. 

 Alexander Monro, The structure and physiology of 

 fishes, Edinb. 1785. F. J. Gall et G. Spurzheim, 

 Anatomic et physiologic du systeme nerveux, Paris, 

 1810. Cumer, Memoires pour servir a 1'histoire 

 et a 1'anatomie des roollusques, Paris, 1817. 

 Geoffroy St. Hilaire, Philosophic anatomique des 

 monstruosites humaines, Paris, 1822. Ftourens, 

 Recherches experimentales sur les proprietes et les 

 fonctions du systeme nerveux, Paris, 1822. Herbert 

 Mayo, Anatomical and physiological commen- 

 taries, second part, July 1823. Wm. HydeWollaston, 

 On semidecussation of the optic nerves, Philoso- 

 phical Transactions, 1824. E. R. A. Serres, Ana- 

 tomie comparee du cerveau, Paris, 1824. Magendie, 

 Journal de physiologic experimental et patho- 

 logique, Paris, 1824. Desmoulins et Magendie, 

 Anatomic des systemes nerveux des animaux et 

 vertebres, Paris, 1825. Frederick Tiedemann, The 

 anatomy of the fretal brain, translated from the 

 French by Wm. Bennett, 1826. Muller, Physiologic 

 des Gesichts-sinnes, Leipz. 1826. Joseph Swan, A 

 demonstration of the nerves of the human body, 

 London, 1830. Frederici Arnoldi, Icones nervorum 

 capitis, 1834. Catalogue of the physiological series 

 of comparative anatomy in the Museum of the 

 Royal College of Surgeons, London, 1833 to 1838. 

 Samuel Solly, The human brain, its configuration, 

 structure, &c. London, 1836. Catalogue of the 

 Museum of the Royal College of Surgeons in 

 Ireland, by John Houston, M.D. Dublin, 1834. 

 Marshall Hall, Lectures on the nervous system and 

 its diseases, London, 1836 : also " Memoirs on the 

 nervous system," by same. Fr. Leuret, Anatomic 

 comparee du systeme nerveux, Paris, 1839. Herbert 

 Mayo, On the chiasma of the optic nerves, 

 London Medical Gazette, November 1841. Arthur 

 Jacob, On paralytic, neuralgic, and other nervous 

 diseases of the eye, Dublin, 1841. John H. Power, 

 Observations on the arrangement of the optic nerve 

 of the loligo, &c. Dublin Journal of Medical 

 Science, 1843. 



(Robert Mayne.) 



ORBIT. (Orbis, any thing round : Fr. tor- 

 bite; GeTm.Augenhohle.) Inthepresentarticle 

 it is intended to describe, first, the bony frame- 

 work of the orbit; secondly, the contents* of the 

 orbit in the order in which they are exposed by 

 a dissection from the roof to the floor of the cavity ; 

 and, lastly, to give some account of the action 

 of the muscles contained in the orbit and in- 

 serted into the upper lid and globe of the eye. 



The orbits are two in number, situated at the 

 anterior and upper part of the face. They 

 have the form of quadrangular pyramids, the 

 bases of which are directed forwards and out- 

 wards, the apices backwards and inwards. 

 Each orbit presents for examination four walls, 

 four angles, formed by the meeting of the walls, 

 a base, and an apex. 



The superior wall or roof is concave and di- 

 rected downwards and slightly forwards. It is 

 chiefly formed by the orbital plate of the frontal 

 bone; at the posterior part to a slight extent by 

 the lesser wing of the sphenoid. It presents 

 the suture between the orbital plate of the 

 frontal and the lesser wing of the sphenoid, and 

 anteriorly on the outer side, the lachrymal 

 fossa, which receives a gland of the same name ; 



f The relative anatomy only of these parts will 

 be given in this article, a special description of the 

 EVE and LACHRYMAL ORGANS having already 

 been given. 



