Sentence:  He deſcribes in a Mechanical way the "manner of its" Action; ſuppoſing that ſome of its Particles having undergone ſeveral <WSD>Comminutions</WSD> (tho not a Diſſolution) from the various Firments they have paſſed, arrive at laſt, with the Blood, at the Brain; ... thoſe of them, which are not ſmall enough to paſs thoſe Straits along with the Juice, being juſt admitted, ſtick there till other appelling Subſtances give them a farther <WSD>Comminution</WSD>, and ſo pretrude them along: ... 
Meaning of the word marked with <WSD> tags:  The breaking or grinding up of a material to form smaller particles.