R中summary(res)["residuals"]和summary(res)$residuals有何不同？

Question

> a
[1] 4 5 6 7
> b
[1] 10 29 32  4
> c
[1] -1  9  8  6
> res=lm(c~a+b)
> summary(res)["residuals"]
$residuals
         1          2          3          4 
-0.8939663  2.2493345 -1.8167702  0.4614020 

> summary(res)$residuals
         1          2          3          4 
-0.8939663  2.2493345 -1.8167702  0.4614020 

summary(res)$residuals和summary(res)["residuals"]有何不同？