578 LECTURE LIX. 



redundant, and such as are required for particular purposes subservient 

 to the animal functions. It is probably in these processes that heat is 

 evolved ; for by experiments on living animals, it has been found, that the 

 Wood, returning from the lungs, is not warmer than before its entrance 

 into them : we must therefore suppose, that when the florid arterial blood 

 is, by some unknown means, converted, in the extreme ramifications of 

 the arteries, into the purple venous blood, to return to the heart by the 

 converging branches of the veins, there is a much more considerable extri- 

 cation of heat, than in the conversion of venous into arterial blood, by the 

 absorption of oxygen and nitrogen in the lungs. If the chyle is actually 

 converted into blood in the lungs, it is here that we must look for the 

 formation of the red globules, those singular corpuscles, to which the blood 

 owes its colour, as it does its power of coagulation to a glutinous lymph, 

 mixed with a less coagulable serum. The red particles in the human blood 

 are about ^Vu- of an inch in diameter, somewhat oblong, and flattened ; 

 they have usually the appearance of a dark point in the centre ; but there 

 is some reason to suspect that this is merely an optical deception. In a 

 few animals they are a little smaller, but in most of the amphibia, much 

 larger and flatter than in man. While the lymph remains fluid, after the 

 blood has been withdrawn from the vessels, these globules tend to subside, 

 and to leave it semitransparent : hence arises the appearance of a buff 

 coat on blood left to coagulate, which is thinner or thicker, accordingly as 

 the globules are sooner or later arrested in their descent. 



The muscles are probably furnished by the blood with a store of that 

 unknown principle, by which they are rendered capable of contracting, for 

 producing locomotion or for other purposes, in obedience to the influence 

 transmitted by the nerves from the sensorium ; the brain and nervous 

 system in general are also sustained, by means of the vascular circulation, 

 in a fit state for transmitting the impressions, made by external objects 

 on the senses, to the immediate seat of thought and memory, in the 

 sensorium ; and for conveying the dictates of the will, and the habitual 

 impulses almost independent of volition, to the muscular parts of the 

 whole frame. 



In what manner these reciprocal impressions are transmitted by the 

 nerves, has never yet been fully determined : but it has long been conjec- 

 tured that the medium of communication may bear a considerable analogy 

 to the electrical fluid ; and the extreme sensibility of the nerves to the 

 slightest portion of electrical influence, as well as the real and apparently 

 spontaneous excitation of that influence in animal bodies, which have been 

 of late years evinced by galvanic experiments, have added very materially 

 to the probability of the opinion. An extremely slender fibre, of a sub- 

 stance capable of conducting electricity with perfect freedom, enveloped in 

 a sheath of a perfect nonconductor, would perhaps serve to communicate 

 an impulse, very nearly in the same manner, as the nerves appear to do. 

 Indeed nothing can be more fit to constitute a connecting link between 

 material and immaterial beings, than some modification of a fluid, which 

 appears to differ very considerably, in its essential properties, from the 

 common gross matter of the universe, and to possess a subtility and 



