Saliva is a watery substance formed in the mouths of animals, secreted by the salivary glands. Human saliva comprises 99.5% water, plus electrolytes, mucus, white blood cells, epithelial cells (which can be used to extract DNA), glycoproteins, enzymes (such as amylase and lipase), antimicrobial agents such as secretory IgA and lysozyme.
Take it on the arches does not mean simply Leave. It means Walk away. If Travis continues to break the chops of his dispatcher, he will not be driving a cab tonight. He will be walking home. I had no idea it was such an obscure term. It is dated, to be sure.
As a prefix, arch- appears in a number of titles referring to positions of superiority, such as archduke and archbishop. Ultimately deriving (via Latin and French) from the Greek verb archein (to begin, rule), it can also mean chief  (as in archnemesis) or extreme (as in archconservative). In the 17th century, as the extreme sense of arch came to be used frequently to describe rogues, knaves, and other clever and mischievous sorts, arch eventually settled into use as an adjective to describe one with impish or playful qualities.