322 
DR. MARSHALL HALL ON THE INVERSE RATIO 
only the other half of that function, viz. the effect of that susceptibility under 
the actual influence of stimuli. The designation irrito-contractility would ex- 
press the whole phenomena. 
Organic life appears to result from the impression of stimuli upon parts 
endued with irritability. The principal stimuli in nature, are air, food, and 
heat ; the principal and corresponding organs of irritability are the heart, the 
stomach, and the muscular system in general. 
The animal series consists of beings variously modified by the varied degree 
of irritability, and by the varied quantity of stimulus. Throughout the whole 
these observe an inverse ratio. The bird tribes and the mammalia are cha- 
racterized by great respiration, whilst the irritability of the muscular fibre is 
low ; the reptiles, the batrachia and the fish tribes, on the other hand, are en- 
dued with a high degree of irritability, and little respiration. The higher parts 
of the zoological series consist of animals chiefly characterized by the appro- 
priation of a great quantity of stimulus ; the lower, by the high degree of irri- 
tability of the muscular fibre. The former are animals of stimulus — of acti- 
vity ; the latter are animals of irritability. 
The due actions of life, in any part of the zoological series, appear to de- 
pend upon the due ratio between the quantity of atmospheric change induced by 
the respiration, and the degree of irritability of the heart : if either be unduly 
augmented, a destructive state of the functions is induced ; if either be unduly 
diminished, the vital functions languish and eventually cease. If the bird 
possessed the degree of irritability of the reptile tribes, or the latter the quan- 
tity of respiration of the former, the animal frame would soon wear out. If, 
on the contrary, the bird were reduced to the quantity of respiration appro 
priate to the reptile, or the latter to the degree of irritability which obtains in 
the former, the functions of life would speedily become extinct. Various de- 
viations from the usual proportion between the respiration and the irritability, 
however, occur, but there is an immediate tendency to restore that propor- 
tion ; increased stimulus exhausts or lowers the degree of irritability, whilst 
diminished stimulus allows of its augmentation. The alternations between 
activity and sleep afford illustrations of these facts. 
Changes in anatomical form in the animal kingdom present other illustra- 
tions of the law of the inverse proportion of the respiration and irritability. 
