The first thought that crossed his mind was to call for assistance.

crossed 是动词 cross 的过去式，意思是"穿过、越过"，这里比喻想法快速闪过。 his 是代词 he 的所有格形式，表示"他的"。 mind 的意思是"头脑、思想"，指人的思维活动或意识。 crossed his mind 是一个固定短语，字面意思是"穿过他的脑海"，引申为"突然想到、闪过念头"。