218 SPECIAL PHYSIOLOGY. 



moth, sometimes characterized as a shriek, is also produced by the friction of 

 parts connected with the mouth and proboscis. 



The remaining and lower members of the animal kingdom, being mostly 

 aquatic, have no vocal or even other special sonorous apparatus. 



SENSATION THE REGULATION OF MOVEMENT 

 THE PSYCHICAL FUNCTIONS. 



NERVOUS EXCITABILITY CONDUCTILITY SEN SIBILITY. 



THE vital property of sensibility, which belongs to the nervous tis- 

 sues, consists in the power of being so excited by various external or 

 internal stimuli, as to produce the phenomena of sensation. But this 

 definition does not express the whole of the vital properties of the 

 nerve-tissues ; nor does it accurately define those which are concerned 

 in the sensory phenomena alone. For example : stimuli applied to the 

 nerves, may not only excite sensation, but may also induce contrac- 

 tions, or motion, in the muscles, accomplishing this, either by the 

 direct conduction of a stimulus along a nerve, or else by the conduc- 

 tion of a stimulus to a nervous centre, whence it is reflected, along 

 another nerve, to the muscles. Again, in the phenomena of sensation 

 itself, it is necessary to distinguish between the excitation of a nerve 

 by a stimulus, its conduction along the nerve, and its final effect upon, 

 or reception by, a nervous centre. 



The kinds of stimuli which will excite, a nerve, are the same as those 

 mentioned in speaking of the muscular contractility, viz. : mechanical 

 stimuli, such as tickling, scratching, pricking or pinching, bruising, 

 stretching, tearing or cutting; the stimulus of heat or cold; irritants 

 and chemical substances, such as mustard, acetic acid, salt, or 'mineral 

 compounds some acting by removal of water, as sulphuric acid or 

 chloride of calcium ; others by abstracting fat, as ether ; and others 

 by solution of the albuminoid substances, such as alkalies. The action 

 of these stimuli on the nerves engaged in sensation, differs ; acids, for 

 example, causing much pain, and but little or no muscular contraction. 

 Light, and also some chemical substances, produce effects without any 

 recognizable change in the nervous substance ; such are the oxygen of 

 the blood, sapid and odorous particles, and certain products of the 

 nutrition or waste of the tissues, as well as many medicinal and poi- 

 sonous substances. Electrical stimuli, whether galvanic, magnetic, or 

 frictional, and even the electrical currents existing in animal tissues, 

 likewise excite the nerves. But nerve is distinguished from muscle, 

 by being excitable through certain stimuli called vital, originating in 

 its own substance, or acting upon it from without, such as the reflex 

 and mental stimuli, which cannot call muscular contractions into play 

 directly, i. c., without the intervention of nervous substance. The 

 excitability of particular nerves is also aroused, in peculiar ways, in 

 the exercise of the special senses, as, for example, taste and smell by 

 chemical action, hearing by vibrations in a surrounding medium, and 

 sight by the undulations which cause the sensation of light. Psychi- 



