VOL, XXXVII.] PHILOSOPHICAL TRANSACTIONS. 551 



Exper. 3. — He tied a piece of fine twine, or thread, parallel to the crural 

 artery, vein, and nerve of a dog; and made a ligature on them, and on the 

 parallel twine, above and below, at the distance of about 4 inches; then he 

 cut beyond the ligatures above and below, so as to take out the vessels and 

 nerve, with the parallel twine, in one bundle; and laying them on a board, 

 both the artery and vein contracted immediately, and were shortened to almost 

 one half of the natural length which they had in the body, viz. to 2\- inches; 

 whereas the nerve remained uncontracted, at its natural length, and commen- 

 surated to the parallel twine of 4 inches, as before it was cut out of the body. 



By which it appears that the proportion of the blood-vessels in their com- 

 pletest contraction, to the same in a state of extension, and to the nerves at 

 their constant and natural length, is nearly as 5 to 8; or, which is the same 

 thing, any given section of a blood-vessel, cut out and left to itself, is capable 

 of contracting, so as to lose 4 parts of its length. 



But though this experiment may suffice for estimating the elasticity of the 

 blood-vessels in general; yet it is not to be doubted, but the degree of their 

 strength and elasticity may differ a little more or less in animals of different 

 species, and individuals of the same species, nay even in the same individual at 

 different stages of life; but these differences are not material to the present 

 purpose, which is only to show that the nerves are not elastic, and that the 

 ijlood-vessels are so to a very considerable degree. 



Inferences from these Experiments. — The first two experiments show, that 

 the brain and nerves contribute to muscular motion, and that in a very high 

 degree. 



The third experiment makes it as plain, that what they contribute in 

 muscular motion, cannot arise from, or be owing to elasticity, which they 

 have not. 



What remains therefore but to conclude, that the action of the nerves in 

 muscular motion, is owing to the fluid they contain, by whatever name we may 

 choose to call it. 



To fortify this conclusion, let us consider, that we can have no other evi- 

 dence of the existence of that invisible fluid the air, and of its several qualities 

 of elasticity and gravity, but what arises from experiments and observations of 

 its effects; which are sufficiently satisfactory, and convince us of its existence, 

 though the minute particles of its composition fall under none of our senses. 

 Therefore, in the same manner, seeing these experiments put the elasticity and 

 elastic vibrations of the nerves quite out of the question, we may as fairly con- 

 clude, that there is a fluid in the nerves, though invisible, as that there is such 

 a fluid called the air, though it cannot be seen. 



