[ 39 ] 
XLVItt. 
The laft Opinion I fhall trouble you with Stuart. 
is that of my Predecefl.br in thefe Lectures, 
the late learned Dr. Stuart 3 who (in his Explanation 
of the Experiment upon the Frog, to which I refer 
the Reader) concludes, “ That voluntary mufcular 
“ Motion is begun by the Impulfe of the Mind or 
<e Will on the animal Spirits, thro' the Nerves into 
tc the Mufcles.” 
He fuppofes (with Keill ) the Structure of a mufcu- 
lar Fibre to be veficular, with a reticular 'Plexus of 
Blood-veflels invefting each Veflcle : His particular 
Definition of it is this: “ It is a nervous Fibre, pro- 
“ duced from its Entrance into the Mufcle, along, 
11 or in the Axis of each carnous Fibre, in the Form 
“ of a Chain of diften file Veflcles, whofe Sides are 
“ cover’d with a Net- work of claftic longitudinal and 
“ tranfverfe Blood-veflels, Here he makes a 
Difference between the nervous Fibre in the Form of 
a Chain of Veflcles, and the carnous Fibre, along 
whofe Axis it is produced ; whereas Dr. Keill fays. 
Each Fibre is a String of Veflcles. Our Author en- 
deavours to explain his Syftem by thefe little longi- 
tudinal and tranfverfe Blood veflels on the Surface of 
each of thefe Veflcles, which he calls the reticular 
Plexus. I muff confcfs I do not well comprehend 
his Meaning ; however, the Sum of his Account is 
as follows : 
“ In the utmofl State of Extenfion (of a Mufcle), 
“ the longitudinal capillary Blood-veflels on the Sur- 
tc face of each Veflcle in the Fibres mud be extended, 
“ and therefore their tranfverfe Diameters mud be 
<c leffened 5 
