CUT 1 XEO f T S A XI) M ( 'S< TLA II SKXS. I TIOXS. 



391 



the object to that of the whole field of vision ; but as objects of different 

 size may fill the same visual augle when at different distances from the eye, 

 our estimate of their size depends upon the distance at which we suppose 

 them to be situated. The distinctness of the surface features of the body 

 afford the mind an important clue, since experience shows that details of 

 surface in a body become more obscure as we recede from that body. But 

 more important data concerning distance come from the sense of muscular 

 innervation, or feeling of the intensity of muscular contraction, by which we 

 estimate the degree of convergence of the optic 

 axes when the object is focussed, and still more by 

 the perception of the amount of muscular effort 

 necessary to sweep the optic axes over the ground 

 surface intervening between the observer and the 

 object. When objects approach the near-point of 

 vision the sense of innervation of the pupillary 

 muscles affords important evidence of their distance. 



That fundamental education concerning the outer 

 world which engages the earliest years of every child 

 consists in accumulating and systematizing with 

 other sense-perceptions tactile and muscular im- 

 pressions of objects. A sensation is no sooner felt 

 than some muscular movement involving a definite 

 muscular feeling is made by which the character of 

 the sensation is changed and experimentally tested 

 under different conditions. The physiological pro- 

 cess involved in building up sense-knowledge, there- 

 fore, embraces in alternation sensation excited by 

 external objects, motion accompanied by muscular 

 sensation, and change in the original sensation. I n 

 other words, the motor and sensory impulses form a 

 sort of balance, and both are necessary. 



Ending- of Sensory Nerve-fibres in the Skin. — 

 The afferent nerves supplied to the skin have several 

 modes of termination. Tn the commonest form the 

 plexus of medullated nerve-fibres found in the dermis 

 close under the epidermis gives off twigs which, losing the medullary sheath, 

 pierce the epidermis and here form a network among the cells of the Mal- 

 pighian layer, the single fibres ending freely in this position (Fig. 207). So 

 numerous are they thai it would appear that every epithelial cell (of mucous 

 membrane as well as skin) is in contact with one or more nerve-fibrils. These 

 axis-cylinder threads are often varicose and usually end freelyamong the 

 epithelial cells, but in some cases they are expanded at their terminations 

 into well-defined sensory end-plates. In the corium and subcutaneous con- 

 nective tissue (mesoblastic) sensory nerves may terminate in the manner just 

 described. But they are frequently modified into or form a part of definite 



Fig. 200.— Magnified view of a 

 Pacinian body from the cat's 

 mesentery (from Quain, after 

 Ranvier): », stalk with nerve- 

 fibre enclosed in sheath of Henle, 

 passing to the corpuscle; »', its 

 continuation through the coil, m, 

 as a pale fibre ; a, termination of 

 the nerve in the distal end of the 

 core (the terminations are not 

 always arborescent); <l. lines 

 separating the tunics of the cor- 

 puscles; /. channel through the 

 tunics, traversed by the nerve- 

 fibre; C, external tunics of the 

 corpuscle. 



