98 HUMAN PHYSIOLOGY. 



Special stimuli : 



1. Light or ethereal vibrations acting upon the end organs of the optic 



nerve in the retina. 



2. Sound or atmospheric undulations acting upon the end organs of the 



auditory nerve. 



3. Heat or vibrations of the air acting upon the end organs in the skin. 



4. Chemical agencies acting upon the end organs of the olfactory and 



gustatory nerves. 



As to the nature of the nerve impulse generated by the above stimuli but 

 little is known. It is supposed to be a mode of motion, molecular or 

 vibratory in character, which passes through the axis cylinder with a definite 

 velocity. 



Rapidity of Transmission of Nerve Force. The passage of a nervous 

 impulse, either from the brain to the periphery or in the reverse direction, 

 requires an appreciable period of time. The velocity with which the 

 impulse travels in human sensory nerves has been estimated at about 190 

 feet per second, and for motor nerves at from 100 to 200 feet per second. 

 The rate of movement is, however, somewhat modified by temperature, cold 

 lessening and heat increasing the rapidity ; it is also modified by electrical 

 conditions, by the action of drugs, the strength of the stimulus, etc. The 

 rate of transmission through the spinal cord is considerably slower than in 

 nerves, the average velocity for voluntary motor impulses being only 33 

 feet per second, for sensitive impressions 40 feet, and for tactile impressions 

 146 feet per second. 



Phenomena of Muscles and Nerves. The muscles are the motor 

 organs of the body and constitute a large per cent, of the body weight. 

 Muscles are of two kinds, striated and non-striated or involuntary. The 

 striated muscles consist of bundles of fibres, the fasciculi, held together by 

 connective tissue. Each muscle fibre is about % to I ^ inches long, and 

 possesses a delicate homogeneous membrane, the sarcolemma, in the interior 

 of which is contained the contractile substance, which presents a striated 

 appearance. During life this substance is in a fluid condition, but after 

 death undergoes stiffening. 



The non-striated muscles form membranes which surround cavities, e.g., 

 stomach, arteries, bladder, etc. They are composed of elongated cells 

 without striations, and contain in their interior one or more nuclei. 



Muscular tissue is composed of water, an organic contractile substance, 

 myosin, non-nitrogenized substances, such as glycogen, inosite, fat, and 



