THE SENSE OF TOUCH 689 



external temperature caused by natural or artificial means, which diminishes 

 the radiation, the temperature of the skin will at once rise, the end-organs 

 will be stimulated, and a sensation of warmth developed. If, on the other 

 hand, there is a sudden fall in temperature and an increased radiation, the 

 temperature of the skin will fall, the end-organs will be stimulated, and a 

 sensation of cold developed. Experiment also teaches that the intensity of 

 a warm or cold sensation will depend on the existing temperature of the skin, 

 and not upon the absolute temperature of the object. Thus, water at 2OC. 

 will evoke a sensation of heat or cold respectively according as the skin 

 has previously been cooled below or warmed above this temperature. 



The Muscle Sense. As a result of the activities of the musculature of 

 the body or even of its individual parts, there arises in consciousness a 

 series of sensations which are termed muscle sensations. These sensations 

 give rise to the perception 



1. Of the direction and duration of both passive (due to external causes) 



and active movements (due to internal, volitional efforts) which take 

 place without hindrance; 



2. Of the resistance offered to movements by external bodies; and 



3. Of the posture of the body or of its individual parts. 



As to the seat of the physiologic processes which precede and underlie 

 the development of the sensations two views, at least, may be advanced, viz. : 



1. That the processes are central in origin and partake of the nature of a 



discharge of nerve impulses from the nerve-cells through the motor 

 nerves to the muscles, the entire process being accompanied by sensation. 

 This is known as the innervation theory. 



2. That the processes are peripheral in origin, initiated by stimulation of 



specialized end-organs in the muscles and tendons which are connected 

 through the intermediation of afferent nerves with nerve-cells in the 

 cerebral cortex. 



pf.c. 



pr.e. 



FIG. 281. A NEURO-MUSCLE SPINDLE OF A CAT. (Ruffini.) c. Capsule, pr. e. Primary 

 ending, s. e. Secondary ending, pi. e. Plate ending. (All these are probably sensor in function.) 

 (Starling's " Physiology.") 



The physiologic mechanism subserving the muscle sense, according to the 

 second theory, now held by many physiologists, thus involves peripheral end- 

 organs, afferent nerves, their cortical connections and nerve-cells in the 

 cerebral cortex at or near the junction of the superior and inferior parietal 

 convolutions. 



The End-organs. These are small fusiform structures found in and 

 among the muscle bundles of all the muscles of the body with the exception 

 of the diaphragm and eye muscles. In the muscles of the arm and in the 

 small muscles of the hand they are especially abundant. From their shape 

 they are known as muscle spindles. They vary in length from 2 to 12 mm, 



44 



