310 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



or other cells around them (Fig. 17.1). Others end in cellular capsules 

 containing a clear jelly. In such capsules the nerve endings can be affected 

 by slight or by heavy pressure without injury. In man and other mammals, 

 pressure receptors of this type, the Pacinian corpuscles, are located around 

 joints and tendons, in fingers and toes, and in deeper parts of the body, 

 e.g., the mesentery, wherever pressure is apt to be. The Pacinian corpuscle 

 is oval; when it is pressed it elongates and the nerve fiber within it is stretched 

 (Fig. 17.1). Nerve endings are twined around the roots of hairs that are 

 sensitive to the slightest pressure, even of air currents, such as, the hairs in 



Meissner's corpuscle 

 Sebaceous gland....; 

 End bulbs of Krause j Hair 



Smooth 



muscle 



t 



Tactile disks Free nerve endings 

 Epidermis • Dermis 



Nerve ending Subcutaneous Pacinian Duct of Ruffini 



around hair fat corpuscle sweat gland ending 



Fig. 17.1. Diagram of the nerves and end organs in a section of human skin. 

 Not all of these endings are to be found in any one area. Cold receptors; Meissner's 

 corpuscles, abundant on the palm side of the fingers. Touch receptors; nerve end- 

 ings entwined about the hairs often with end feet applied to them; highly developed 

 association with cat's whiskers. Pressure and possibly vibration receptors; Pacinian 

 corpuscles, abundant in skin of palms of hands and feet, internal organs and 

 mesentery. Heat receptors; Ruffini endings lie deep in the skin. Receptors for pain 

 are not shown; they consist of bare nerve cell fibers extending about and between 

 cells. The number of structures present in a small area of skin may be realized 

 in connection with the sweat glands one of which is shown in this diagram. By 

 counts of limited spaces at least 3000 sweat glands are calculated to be in the 

 skin of the palm of the hand. (Courtesy, Gardner: Fundamentals of Neurology, 

 ed. 2. Philadelphia, W. B. Saunders Co., 1952.) 



