83O SENSORY AND TACTILE SENSATIONS. 



tud-flakes or plates in the sterno-radial muscle of the frog, or elongated oval end-bulbs, not un- 

 like the end-bulbs of the conjunctiva, or small simple Pacinian corpuscles.] 



Prus found ganglion cells more frequently in the subcutaneous tissue than in the corium, 

 and they appeared to have some relation to the blood-vessels and sweat-glands. 



425. SENSORY AND TACTILE SENSATIONS. In the sensory nerve- 

 trunks there are two functionally different kinds of nerve-fibres : (1) Those which 

 administer to painful impressions, which are sensory nerves in the narrower sense of 

 the word ; and (2) those which administer to tactile impressions and may therefore be 

 called tactile nerves. The sensations of temperature and pressure are also reckoned 

 as belonging to the tactile group. It is extremely probable that the sensory and 

 tactile nerves have different end-organs and fibres, and that they have also special 

 perceptive nerve-centres in the brain, although this is not definitely proved. This 

 view, however, is supported by the following facts : 



1. That sensory and tactile impressions cannot be discharged at the same time 

 from all the parts which are endowed with sensibility. Tactile sensations, includ- 

 ing pressure and temperature, are only discharged from the coverings of the skin, 

 the mouth, the entrance to and floor of the nose, the pharynx, the lower end of the 

 rectum and genito-urinary orifices ; feeble indistinct sensations of temperature are 

 felt in the oesophagus. Tactile sensations are absent from all internal viscera, as 

 lias been proved in man in cases of gastric, intestinal, and urinary fistulee. Pain 

 alone can be discharged from these organs. 2. The conduction channels of the 

 tactile and sensory nerves lie in different parts of the spinal cord ( 364, 1 and 5). 

 This renders probable the assumption that their central and peripheral ends also 

 are different. 3. Very probably the reflex acts discharged by both kinds of nerve 

 fibres the tactile and pathic are controlled, or even inhibited, by special central 

 nerve-organs ( 361 ?). 4. Under pathological conditions, and under the action of 

 narcotics, the one sensation may be suppressed while the other is retained ( 364, 5). 



Sensory Stimuli. In order to discharge a painful impression from sensory 

 nerves, relatively strong stimuli are required. The stimuli may be mechanical, 

 chemical, electrical, thermal, and somatic, the last being due to inflammation or 

 anomalies of nutrition and the like. 



Peripheral Reference of the Sensations. These nerves are excitable along their 

 entire course, and so is their central termination, so that pain may be produced by 

 stimulating them in any part of their course, but this pain, according to the " law 

 of peripheral perception," is always referred to the periphery. 



The tactile nerves can only discharge a tactile impression or sensation of contact 

 when moderately strong mechanical pressure is exerted, while thermal stimuli are 

 required to produce a temperature sensation, and in both cases, the results are 

 obtained only when the appropriate stimuli are applied to the end-organs. If 

 pressure or cold be applied to the course of a nerve-trunk, e.g., to the ulna at the 

 inner surface of the elbow-joint, we are conscious of painful sensations, but never of 

 those of temperature, referable to the peripheral terminations of the nerves in the 

 inner fingers. All strong stimuli disturb normal tactile sensations by over-stimula- 

 tion, and hence cause pain. 



The law of the specific energy of nerves leads us to assume that the cutaneous 

 nerves contain different kinds of nerve-fibres with different kinds of end-organs, 

 which subserve different kinds of impressions, e.g., pressure, temperature, and pain. 

 lUix and Goldscheider have found such differences. Electrical stimulation causes 

 different sensations according to the part of the skin where it is applied; at one 

 spot, pain only is produced, at another a sensation of cold, at a third a sensation of 

 neat, and at a fourth, a sensation of pressure. At every temperature point or spot, 

 there is insensibility for pain or pressure. The " pressure-points " or pressure-spots 

 he much closer together, and are more numerous than the temperature-points. 

 Inere are special "pain-spots" and even "tickling-spots." These spots are 



