14 STRUCTURE OF THE EYE. 



undergo the exposure to heat or cold ; for a similar effect was produced 

 when the trunk of the nerve was acted on, The ulnar nerve is the one 

 best suited to illustrate this fact, its trunk lying immediately beneath the 

 surface at the elbow. After immersing the elbow in a mixture of ice and 

 water for about sixteen seconds, Weber observed that a peculiar painful 

 sensation was perceived along the under side of the fore-arm, the wrist, the 

 little finger, and the inner side of the ring finger. The pain had no 

 resemblance to that of cold. On continuing the immersion the pain 

 increased considerably, and eventually became almost intolerable ; then it 

 gradually diminished, and the little and the ring fingers became numb, as 

 if '" asleep," had no longer the power of distinguishing between heat and 

 cold, and could only imperfectly perceive the contact and pressure of bodies. 

 The sense of smell also would appear, from Weber's experiments, to be for 

 the time suspended, after the cavity of the nose has been filled with either 

 hot or cold water. But the influence of heat and cold is, in this case, less 

 certain, because the action of water alone, independent of its temperature, 

 on the mucous membrane of the nostrils, will for a time suspend the sense 

 of smell. 



OF THE SENSE OF VISION.* 



Tunics of the eye. The Tapetum.\ As will be mentioned again when 

 noticing the structure of the retina, M. Brlicke is of opinion that the staff- 

 shaped bodies composing the so-called membrana Jacobi, probably serve the 

 purpose of returning to the sensitive portion of the retina those rays of 

 light which have traversed the retina, and which are not absorbed by the 

 pigment of the choroid. M. BruckeJ believes that this is peculiarly the case 

 in those animals provided with a tapetum ; and he considers that the func- 

 tion of the tapetum is to reflect the light on the staff-shaped bodies situated 

 over that part of the retina most used in vision, and so to enable these 

 animals to see, at times when animals unprovided with a tapetum would be 

 in darkness. He observes that all the colours emitted in the dark from the 

 eyes of animals possessed of a tapetum proceed from this structure alone, 

 except the red, which is produced entirely by the blood in the large vessels 

 of the retina and choroid. Hitherto the tapetum has been described as 

 consisting, in all cases, of numerous undulating smooth and transparent 

 fibres, so arranged as to form a fine membrane. But Briicke finds that 

 although this, which he calls the fibrous tapetum, exists in most ruminants, 

 Solidungula, and the whale tribe, yet in Carnivora this fibrous struc- 

 ture is replaced by another, composed entirely of smooth, nucleated, some- 

 what hexagonal-shaped cells ; and this he calls a cellular tapetum. These 

 cells vary from -0008 to '0028 of an inch in diameter ; by transmitted light 



* Book the Fifth, section i. p. 1088, Miiller's Physiology. f Pago 1119. 



J Miiller's Archiv. 1 845, p. 388. 



