21 



since it convinces us that spasms cannot exist in 

 the greater arteries, and that all the aberrations 

 in the circulation of the blood, generally ascribed 

 to this cause, altogether relate to the unquestion- 

 ably muscular heart, with its auricles, and in 

 some degree to the muscular fibres, which sur- 

 round the extremities of the venae cavse. 



I mentioned, that the final ramifications of 

 arteries, on account of their fineness, are called 

 capillary vessels. The anatomy of these vessels 

 is almost unknown, and the manner in which 

 they terminate is a complete secret. It has hi- 

 therto been impossible to institute any chemical 

 analysis of their integuments, as they cannot be 

 separated from those parts with which they are 

 interwoven. These vessels, in all probability, have 

 a peculiar power slowly to carry forwards the 

 humours contained in them; but the mechanism 

 of this process will probably long remain a se- 

 cret. It is in these vessels that the inexplicable 

 processes of secretion and re-production of solids 

 is performed by means of the co-operation of the 

 nerves. The same nature, which is incompre- 

 hensible to us, when extended to immensity, es- 

 capes no less our penetration, when contracted 

 jnto too narrow limits. At both these extremi- 



