Quote 7
He threw the sinner down, then wheeled along
the stony cliff: no mastiff’s ever been
unleashed with so much haste to chase a thief. (Inf. XXI, 43-45)
Already inhuman by nature, this demon confirms bestiality when his savagery towards the struggling sinner spurs Dante to compare him to a fierce guard dog, a mastiff. His "unleashed" behavior suggests that, like the incontinent sinners, the demon is unable to contain his baser instincts.
Quote 8
As I kept my eyes fixed upon those sinners,
a serpent with six feet springs out against
one of the three, and clutches him completely.
It gripped his belly with its middle feet,
and with its forefeet grappled his two arms;
and then it sank its teeth in both his cheeks;
it stretched its rear feet out along his thighs
and ran its tail along between the two,
then straightened it again behind his loins.
No ivy ever gripped a tree so fast
as when that horrifying monster clasped
and intertwined the other’s limbs with its.
Then just as if their substance were warm wax,
they stuck together and they mixed their colors,
so neither seemed what he had been before; (Inf. XXV, 49-63)
The thieves’ punishment of being transformed into hideous beasts (like "a serpent with six feet") reflects their lack of respect for boundaries. Because in life, they did not recognize other people’s property as off limits, they are punished with a violation of their own physical boundaries; they must become one another, sticking together and "mix[ing] their colors" like "warm wax." Dante considers the ability to distinguish between one’s property and another’s a purely human one, not recognized by beasts; thus, by failing to respect boundaries, the thieves forfeit their humanity and become beasts, both physically and spiritually.
Quote 9
…his tongue, which had before been whole and fit
for speech, now cleaves; the other’s tongue, which had
been forked, now closes up; and the smoke stops.
The soul that had become an animal,
now hissing, hurried off along the valley;
the other one, behind him, speaks and spits…
…"I’d have Buoso run
on all fours down this road, as I have done." (Inf. XXV, 133-141)
This passage illustrates, in a very visceral way, Dante’s idea that language is a purely human phenomenon. The thief that mutates into a snake has his tongue split in two so that it is no longer "fit / for speech" and can only hiss as he slithers away. The sinner who has exchanged his serpent form for a human one, however, now possesses a whole tongue and commences to speak articulately.