144 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



action, e. g. light falling on the retina of the eye of a frog causes a negative 

 variation of the current of rest of the optic nerve. 



F. CHEMISTRY OF MUSCLE AND NERVE. 



I. CHEMISTRY OF MUSCLE. 



Muscles contain about 75 parts water and 25 parts solids; nearly 21 parts 

 of the solids are proteids, the remaining 4 parts consisting of fats, extractives, 

 and salts. 



Little is known concerning the chemistry of living muscle ; the instability 

 of the complex molecules which makes possible the rapid development of energy 

 peculiar to muscles renders exact analysis impossible. The manipulations 

 essential to chemical analysis necessarily alter and kill the muscle protoplasm. 



Death of the muscle is ordinarily associated with a peculiar chemical change 

 known as rigor mortis. To understand the chemical composition of muscle it 

 is necessary that we should consider the nature of this change. 



1. Rigor Mortis. Rigor mortis, the rigidity of death, is the result of a 

 chemical change in the substance of a muscle by which it is permanently 

 altered, its irritability and other vital properties being irretrievably lost. The 

 change is manifested by a loss of translucency, the muscle becoming opaque, and 

 by a gradual contraction, accompanied by a development of heat and acidity > 

 and resulting in the muscle being stiff and firm to the touch, less elastic, and 

 less extensible. Whenever muscle dies it undergoes this change. 



Conditions which Influence the Development of Rigor. Ordinarily on the 

 death of the body the muscle enters into rigor slowly the muscle-fibres are 

 involved one after the other, and through the gradual contraction and harden- 

 ing of the antagonistic muscles the joints become fixed and the body acquires 

 the rigidity which we associate with death. Eigor usually affects the different 

 parts of the body in a regular order, from above downward, the jaw, neck, 

 trunk, arms, and legs being influenced one after the other. The position taken 

 by the body is generally determined by the weight of the parts and the rela- 

 tive strength of the contractions of the muscles. 



The time required for the appearance of rigor is very variable. It is deter- 

 mined in part by the nature of the muscle, its condition at the moment of 

 death, and the temperature to which it is subjected. The muscles of warm- 

 blooded animals enter into rigor more quickly than those of cold-blooded 

 animals ; of the warm-blooded animals, pale muscles more quickly than red, 

 and the flexors before the extensors ; of the cold-blooded animals, frog's muscles 

 more quickly than those of the turtle. In general, the more active the muscle 

 protoplasm, the more rapid are the chemical changes which it undergoes, and 

 amongst these the coagulation of rigor mortis. 



The condition of the muscle plays a very important part in determining the 

 onset of rigor. If the muscles are strong and vigorous and death of the body 

 has come suddenly, rigor develops slowly ; if the muscles have been enfeebled 

 by disease or fatigued by great exertion shortly before death, it comes rapidly. 



