TRANSACTIONS OF THK SECTIONS. 67 



working the functions of secretions, and that in the instances of tears, saliva from 

 conceptions of food, and many other instances well-known to physiologists, that minute 

 arteries are excited by retransmissions from conceptions. 



May not the flow of blood through the capillaries be accelerated by the electricity 

 evolved from the chemical affinities of oxygen with the carbon ? 



In the year 1792, while making experiments on frogs and rabbits, and some experi- 

 ments with zinc and silver suggested by Galvani's discovery, I divided the nerve of 

 one of the legs and tied the crural arteries of the others ; the muscles whose arteries 

 were tied soon lost their contractility, while those whose nerves were divided, but 

 whose arteries were not compressed, were excitable for months after the nerve had 

 been divided. From these facts I inferred that the blood and not the nerves influ- 

 enced communication by the brain, and was the source of both sensibility and con- 

 tractility. The frogs were kept in a large pan of water renewed every day, and their 

 skins as little injured as could be avoided ; but when the skin was lightly brushed so 

 as to excite the sensitive extremities of its nerves, a blush was seen on its surface, and 

 the muscles were excitable by zinc and silver in contact with the trunk of the nerve 

 and with each other. 



This appeared to me then, as now, a proof that both sensibility and contractility 

 were communicated analogous, as it now seems, to the sensitiveness communicated to 

 Talbotype paper by chemical preparation. May it not be by the blood projected to 

 the eyes of cats, owls, and all animals who seek their prey in the dark, that the retina 

 is rendered sufficiently sensitive to the smallest degree of light ? 



The late Sir William Herschel says, in one of his astronomical papers, that he always 

 sat in a moderate light, and without moving his eyes, so that the retina might recover 

 its sensibility before he looked into his telescope. We grope our way from a bright 

 sunshine to a diorama, but all is light when we return, and the sensibility of the 

 retina has been revivified by the blood, and the absence of exhausting light. As it is 

 with the eyes, so I infer that it may be with the brain, the organ employed by the 

 mind to effect the thinking functions. 



Blood, says Sir A. Cooper (Guy's Reports), was seen to flush the surface of the brain 

 (perceivable from the loss of a part of the skull and dura mater) with every change of 

 thought, even the most indifferent ; and any one may have observed that the scalp 

 is overheated and the brain sensitive of an accelerated circulation in it when the mind 

 has been long and intently thinking, that with every thought there is a retransmission 

 or projection of blood, not only to the brain, but also to the part whose functions are 

 required for action. 



We have proofs in such cases as those described by Dr. Yellowly in the Medical 

 and Chirurgical Transactions, and others, so ably commented on by Sir Henry 

 Holland. 



The sensibility communicated by the blood in a like case appears to me the efficient 

 cause of consciousness. I have thus far spoken with reference to the red arterial 

 blood only. The venous black blood injected into the brain by Bichat. destroyed life ; 

 and Sir A. Cooper could also suspend all its phenomena by pressure on the carotid and 

 vertebral arteries. Now since all the blood in patients in cholera is black, how is it 

 that their consciousness is not suspended ? Mr. Magendie, in his able pamphlet on 

 cholera, says that the intellect of one patient continued clear for more than two hours 

 after the pulse in the wrist had ceased to beat. I asked him how he reconciled this 

 fact with those recorded by Bichat ; he answered, " My friend Bichat, if living, would 

 have to write that paper over again." May not the following aid our conception of 

 two facts so seemingly incompatible? The skull cannot probably contain more blood 

 at one time than at another, but the proportion of the venous blood may be abnormal, 

 and by its congestion and pressure (as the finger on the denuded brain of the beggar) 

 render a patient comatose. In cliolera there is no pressure by venous blood, for all 

 the fluid parts of the blood have been discharged from the bowels. 



That conceptions are more vivid when we are in such a state of excitement 

 as to accelerate the circulation of the blood in the organs in which conceptions are 

 produced, as in emotions, passions and intensely pleasurable or painful sen- 

 sations, cannot but have been noticed by all who can and do give their attention to the 

 operations of their own minds. The painter seems to see on his canvas such a con- 

 ception of the face ; he by trying to paint the lover sees " his mistress where she has not 

 been," and such conceptions are the object of most illusive appearances. Appear- . 



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