63311
Consider this sentence

> speaker is talking to himself about how to deal with an adversary (X), he
> has so far tried to hire a 暴漢 to beat up X, and a 探偵 to snoop on X, both of
> which have failed.
>
> **暴漢に襲わせ** ても返り討ち、探偵を雇っても不発……。

in this case would interpret this as:

  * (俺が)暴漢に(Xを)襲わせても返り討ち、探偵を雇っても不発……。

Compared to

> 傾いた太陽の橙色が辺り一帯を神々しく染め上げて、風で舞った花びらたちがまるで雪のように煌めいている。
>
> その中心で美しく長い **髪を風になびかせ** ながら、優しさに満ち溢れた瞳を細めて俺を見つめる女の子。

and

> だが、どんなに身をよじって暴れても、男の人の力には敵わなかった。
>
> **興奮に目を血走らせた** おじさんが、まるで嘲笑うようにべたついた手で....

At first glance it would appear

風 "causing" 髪をなびかせる - i don't think 髪を風に(して)なびかせる applies here

興奮 "causing" 目を血走らせた

As far as i can tell, that does not seem grammatical. But i can't easily add
unspoken (...) to these for them to make sense.

compare to:

> 冷たい風が吹き抜け、短い髪をなびかせる。

where 風 is no doubt 短い髪をなびかせる'ing.

How is に and the following causative expressions used here?

thanks
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2018-12-08T19:31:25.443
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question
particle-に causation
に indicating the source of causation?
Both なびく and 血走る are " _intransitive_ verbs that do not take を" when used
plainly. When you form the causative form from these verbs, you have to mark
the agent (causee) with **を** , not に (See [this
rule](https://japanese.stackexchange.com/a/33515/5010)). Here に plays a role
that has nothing to do with the grammar of causation (I believe you already
know [this is 原因の「に」](https://japanese.stackexchange.com/q/62567/5010)).

```

        髪がなびく。            目が血走る。     (simple intransitive usage)
    風に髪がなびく。      興奮に目が血走る。     (simple intransitive usage with に)
        髪をなびかせる。        目を血走らせる。 (causative usage)
    風に髪をなびかせる。  興奮に目を血走らせる。 (causative usage with に)
    
```

Don't confuse this with causative forms made from transitive verbs, where the
agent is marked with に (e.g. 息子にお菓子を食べさせる, 暴漢に奴を襲わせる).
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answer