maybe that's MLN. under steady state the MLN is very small and there is a chance you cannot find it. With infection or inflammation it will become more visible. There are two on the right (of the animal) and one on the left. with a diameter of 0.1~0.2mm, it is a whitish semi-tranparent tiny round ball. The biggest one is on the right beneath the heart on the major blood vessel. the it can be embedded in fat tissues. You may want to carefully pull the whole thing out (sometimes even with the thymus) and then clean the fat/connective tissues before you can see it clearly.

Hi Huizhong, Thank you for your suggestion. I will find it more carefully next time.

This attached paper mentions the MLN of your interest. Hopefully it'll be helpful.

Hi Nguyen and Gustavo, thank you guys very much for your great papers! It is very helpful!

Here is a diagram of murine lymph nodes. Also an article that explains a method of locating them for the first time. Good luck!