386 INTERNAL ORGANS OF SENSE. 



touch most especially resides, presents ridges and grooves 

 of an arched or curved form ; each ridge is a row of conical 

 papillae, and in the grooves the exhalant orifices are found 

 to open. The minute structure of the papillae consists of 

 blood vessels, nerves, and a spongy erectile tissue, and is 

 believed to be exactly analogous with those of the tongue. 

 The vessels are seen as convoluted loops, but the precise 

 termination of the nerves, whether in loops, plexuses, or 

 bulbs, is not yet settled. 



That the papillae, however, are exceedingly sensitive, and 

 constitute the essential seat of touch, there is no doubt. 



The mucous membrane of the eyelids, nose, mouth, 

 larynx, trachea and lungs, is sensible to heat and cold, 

 while that of the vagina, rectum, and urethra, is in addi- 

 tion sensible also to touch. 



CHAPTER Y. 



INTERNAL ORGANS OF SENSE. 



THE internal organs of sense comprise the cerebro-spinal 

 axis, by which is understood the brain and spinal marrow. 

 These organs are the source of sensation and motion to all 

 the organs we have been examining, as those belonging to 

 the mouth and face and external parts of the head. They 

 constitute the foundation and main-spring of all their ac- 

 tions, and without them the functions of these subordinate 

 organs could not be exercised. The examination of the 

 brain and spinal marrow, then, as the source of power to all 

 these parts, seems to follow very naturally, and comes next 

 in the order of investigation we have thought best to 

 pursue. 



This will make complete the anatomy of the head, and 

 carry out the design in view, which is to endeavor to illus- 

 trate as far as possible, practical physiology, by bringing or- 

 gans together, however dissimilar in structure, which have 

 a relationship or community of action in the discharge of 

 any particular function or series of functions ; and thus 



