PHYSIOLOGY. 39 



extremities. We find from the centre of the optic nerve a 

 considerable artery arising, which is distributed in a very sin- 

 gular ramification, so that it would appear that a portion of an 

 artery accompanies every separate fibril of the retina. In like 

 manner, we know that blood-vessels accompany the auditory 

 nerves in their expansion in the vestibulum, cochlea, &c. ; in 

 the nose also a numerous plexus of blood-vessels is every where 

 intermixed with the nervous extremities : hence haemorrhagies 

 are so easily produced there. With regard to the tongue and 

 the skin, it is no less remarkable. In every separate papilla 

 anatomists have traced an artery immersed into its substance, 

 and ramified in its extremity to a great degree of subtlety. 

 Now, admitting the fact that arteries every where accompany 

 the nervous extremities, we further know, that every artery is in 

 a state of tension, i. e. it is filled to a dimension beyond what it 

 would assume if left to itself. But this tension of the blood- 

 vessels is varied upon different occasions ; and therefore the 

 tension which they give to the nervous fibres must be also vari- 

 ous. There is no doubt that such a tension is produced, but 

 it remains to be proved that it produces this effect : but that 

 is not more difficult. The sensibility is occasionally increased 

 in the retina ; but in no instance so remarkably as in the case 

 of inflammation of the eye, especially in those inflammations 

 which affect the arteries in the posterior part of the eye, in con- 

 sequence of which, the sensibility is often increased to a surpris- 

 ing degree. The sensibility of my eye at present is so moderate, 

 that I can destroy it entirely by shutting the eyelids ; and if in 

 some persons this is not sufficient, they can, by the application of 

 a certain number of cloths, be made insensible to the presence or 

 absence of light. But I have known cases of ophthalmia, in 

 which the light could not be borne, even when six or eight folds 

 of linen covered the eye, and the curtains of the bed were drawn 

 round in a dark chamber. The patients would be sensible ta 

 the putting aside the curtain ; and this is only to be explained 

 in this way : that the inflammation is attended with an over- 

 distention of the arteries, which increases the sensibility of the 

 nerves by increasing the tension in the manner we have ex- 

 plained. In other cases, those of headaches approaching to a 

 phrenitic affection, the sensibility both of ears and eyes is so 



