CHAP. XIV.] 



TACTILE PAPILLAE. 



411 



Fig. 82. 



there is good reason to suppose it formed of an unbroken expan- 

 sion of the homogeneous basement membrane already spoken 

 of. Within this it is difficult to distin- Fig. si. 



guish 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, filaments of ex- 

 treme delicacy, which seem to be of the 

 elastic kind, are generally discoverable in 

 it. Injection of the blood-vessels demonstrates the existence of a 

 small arterial twig derived from the arterial plexus of the cutis, en- 

 tering at the base, advancing up the interior of the papilla, and 

 subdividing into two or more capillary vessels, according to the size 

 of the particular organ. These, after forming small loops, reunite 

 either at the base of the papilla, or in the subjacent texture, into 

 small veins, which empty their blood into the venous plexus of the 

 cutis. The capillaries of the smaller pa- 

 pillae frequently join with those of the fat- 

 vesicles that lie beneath. The vascularity 

 of the papillae is such, that their presence 

 and relative size may be determined simply 

 by the depth of the colour imparted to a 

 portion of skin by a good injection of its 

 vessels. The vascularity of the integument 

 is, therefore, in general terms, proportioned 

 to its perfection as an organ of touch. 



Since the discovery of the papillae as gS3S? g vein - ~ Magnified 80 

 the sentient organs, the existence of 



nerves within them has usually been taken for granted, or they 

 have been loosely styled expansions of the nerves; and to the 

 general truth of such statements we may readily assent. But we 

 have reason to doubt the accuracy of some recent writers, who 

 have professed to give minute details of the mode of termination 

 of the nervous tubules in the papillae. The subject is difficult 

 of investigation. According to Ernst Burdach* and others, the 

 nerves are arranged in a plexiform manner under the skin of the 

 frog, and loops are formed by the union of tubules from neighbour- 

 ing branches. On examination we find this description correct as 

 far as it goes, but that it does not carry us to the papillary struc- 

 ture. The plexus in question is situated underneath an expansion 

 of fibres crossing each other at right angles, which is itself placed 

 * Beitrag zur Mikroskop. Anat. der Nerven: Konigsb. 1837. 



Vessels of papillae, from the heel : 

 a. Terminal arterial twig, v. 



