You're using [this book](https://www.amazon.co.jp/Atlas-Anatomy-Artists-Dover/dp/0486202410), right? I liked the details that Fritz drew, but I found Bridgeman's constructive anatomy to be a lot better for perspective and proportions. You should try it.
An Atlas of Anatomy for Artists is an excellent resource. My life drawing class used this.
An Atlas of Anatomy for Artists (Dover Anatomy for Artists) https://www.amazon.com/dp/0486202410/ref=cm_sw_r_cp_apa_rGB2Bb9G5VZ1B This was first, a little gross but important. With a proper understanding of anatomy you can start to form your own creatures and understand how others work.