ORGANS OF THE SENSES. 493 



among the other tissues of the papilla about half way to 

 their summits, but then becoming lost to sight, either by 

 simply ending, or else by losing the white substance of 

 Schwann, which alone enables us to distinguish them in 

 such situations from other textures. Thin vertical sections of 

 perfectly fresh specimens are essential for this investigation, 

 and the observer should try upon them the several effects of 

 acetic acid and solution of potass. In thus describing the 

 nerves of the papilla? from our own observations, we do not 

 deny the existence of true loop-like terminations as figured 

 by so respectable an authority as Gerber ; but neither do we 

 feel entitled to assent to it. ... We incline to the belief 

 that the tubules, either entirely or in a great measure, lose 

 the white substance when within the papillae." 



Messrs. Todd and Bowman further observe, in reference 

 to the structure of the papilla? : " Within the basement 

 membrane it is difficult to distinguish any special tissue, 

 except by artificial modes of preparation. A fibrous struc- 

 ture, however, is apparent, having a more or less vertical 

 arrangement; and with the help of solution of potass, fila- 

 ments of extreme delicacy, which seem to be of the elastic 

 kind, are generally discoverable in it." 



The blood-vessels of the papilla? consist of single loops ; 

 each of these is made up of an artery and a vein : the former, 

 derived from the arterial plexus of the cutis, ascends the 

 papilla on one side, and on reaching its summit gradually 

 merges into the vein which descends along its opposite side, 

 and terminates in the venous plexus of the cutis. In an 

 injected preparation, and where the villi are large, the turn 

 of the loop is seen to be very abrupt, the two vessels, the 

 artery and the vein, coiling round each other, and resembling 

 a piece of twine which has been bent upon itself and afterwards 

 twisted in a spiral manner. This disposition of the vessels 

 seems intended to delay somewhat the passage of the blood 

 through the papillae. (See Plate LXIILjfy. 7.) 



The thickness of the epidermis which so closely invests 

 the papillae, does not appear to have any direct relation to 

 the sense of touch: thus, over the general surface of the body, 



R R 2 



