108 ESSENTIALS OF PHYSIOLOGY. 



end-organ, where it ends in one or more knob-like extremities. Such 

 endings are the end-bulbs found in the conjunctiva, penis, clitoris, and 

 in the synovial membrane of some joints : the touch corpuscles of Meissner 

 in the connective-tissue papillae of those parts of the skin which are 

 most sensitive to pressure, that is, in the hand and front of the forearm, 

 the lips, the foot, and the mammary papilla : the Pacinian corpuscles 

 found in the subcutaneous tissue of the hand, foot, and genital organs. 

 and in the mesentery and some organs, such as the pancreas : the 

 corpuscles of Golgi and Mazzoni and the corpuscles of Ruffini, found in 

 the subcutaneous tissue of the fingers. 



The Sensory Spots in the Skin. If a cooled metal point is drawn 

 gently along the skin of the forearm or back of the hand, a sensation of 

 cold is produced at certain definite spots, which may be marked out 

 with coloured ink. If the point is heated and the experiment repeated, 

 it is found that other spots, quite distinct from those for cold, respond 

 and give the sensation of heat. Similarly, by using the prick of a 

 needle as the stimulus, another series of spots may be marked out 

 which give rise to pain. Lastly, by testing the pressure sense 

 with a bristle a fourth set of spots may be located. The most numerous 

 spots in the skin are those for pain ; next in order are the pressure 

 spots, those for cold being less numerous, and the heat spots fewest of 

 all. The specific character of the different nerve endings is confirmed 

 by the effect of other stimuli. The application of menthol to the skin, 

 for example, stimulates especially the nerve endings for cold and gives 

 rise to a sensation of coolness. If the arm be held in a jar of carbonic 

 acid gas, on the other hand, there is a sensation of warmth, the nerve 

 endings for heat being especially affected. Electrical stimulation of 

 the various spots excites a specific sensation, namely, warmth in a 

 heat spot, pain in a pain spot, and so on. Additional evidence as to 

 the independence of the sensations is derived from the effect of cocaine 

 on the surface of the eyeball. Under normal conditions suitable 

 stimulation of the conjunctiva excites the sensations of heat, cold, or 

 pain, whereas stimulation of the cornea gives rise to the sensation of 

 pain only. The application of cocaine to the eye paralyses the nerve 

 endings for pain, but does not affect those for heat or cold. 



The Conduction of Cutaneous Impulses. The nerve fibres which 

 convey the cutaneous impulses are bound together in common trunks. 

 Prolonged pressure on these trunks acts as a block to the conduction of 

 impulses, and occasionally cases occur in which, by the pressure of a 

 bony outgrowth or otherwise, a partial block is produced, whereby the 

 pressure and temperature senses are lost, while that of pain is retained, 

 or vice versa. A similar partial block occurs at times in the disease 



