224 On muscular Motion. 



removins: the limb into a diluted solution of vegetable al- 

 kali, the muscles were relaxed, but no signs of irritability 

 rttLirned, 



No. 3. under all the former circumstances, retained its 

 previous appearances, and was irritable, but less so than 

 No. 1. 



No. 4. had become rigid, and the final contraction had 

 taken place. 



Other causes of the loss of muscular ir'ritability occur in 

 pathological testimonies, some examples of which may not 

 be ineligible for the present subject. Workmen whose 

 hands are unavoidably exposed to the contact of white lead, 

 are liable to what is called a palsy in the hands and wrists, 

 from a torpidity of the muscles of the fore arm. This affec- 

 tion seems to be decidedly local, because, in many instances, 

 neither the brain nor the other members partake of the 

 disorder; and it oftenest affects the right hand. An inge- 

 nious practical chemist in London has frequently expe- 

 rienced spasms and rigidity in the muscles of his fore arms^ 

 from affusions of nitric acid over the cuticle of the hand 

 and arm. The use of mercury occasionally brings on a si- 

 milar rigidity in the masseter muscles. 



A smaller quantity of blood flows through a muscle daring 

 the state of contraction than during the quiescent state, as 

 is evinced by the pale colour of red muscles when contracted. 

 The retardation of the flow of blood from the veins of the 

 tore arm, during venaesection, when the muscles of the 

 litiib are kept rigid, and the increased flow after alternate 

 relaxations, indirces the probability, that a temporary retarda- 

 tion of the blood in the muscular fibrils takes place during 

 each contraction, and that its free course obtains again during 

 the relaxation. This state of the vascular system in a con- 

 tracted muscle, does not, however, explain the diminution 

 of its bulk, altho\igh it may have some influence on the 

 limb of a living animal. 



When muscles are vigorously contracted, their sensibility 

 to pain is nearly destroyed : this means is employed by 

 jugglers for the purpose of suffering pins to be thrust into 

 ihe'calf of the leg, and other muscular parts, with impu-^ 

 nity : it is, indeed, reasonable to expect, a priori, that the 

 sensation, and the voluntary influence, cannot pass along 

 the nerves at the same time. 



In addition to the influences already enumerated, the hu- 

 man muscles are susceptible of changes from extraordinary 

 occurrences of sensible impressions. Long continued at- 

 tention to interesting visible objects, or to audible sensa- 

 tions. 



