My dear Darwin

Pardon me for not replying to your note at once;¹ I have not been able to put my hand upon a dissection of the muscle in question which I preserved.

However that does not matt⁠⟨⁠e⁠⟩⁠r much for it ⁠⟨⁠tur⁠⟩⁠ned out to be no new discovery being as I soon found out described by Chauveau in his “Anatomie des Animaux domestique,” under the name of (I think) “Detrusor fæcis”² This was the only book in which I found any notice of it   It occurs in all the domestic animals I have examined. It consists of a large flat g⁠⟨⁠    ⁠⟩⁠d from the longitudinal muscular coat of the rectum and as it passes backwards becomes reduced to a narrow band or cord which may be tendinous and sometimes double, as, I think, in the cat. It finally gets inserted into the ventral surface of the body of a caudal vertebra at some distance beyond the anus.

It would appear to be brought into ⁠⟨⁠a⁠⟩⁠ctio⁠⟨⁠n⁠⟩⁠ during the act ⁠⟨⁠    ⁠⟩⁠ when the animal ⁠⟨⁠s⁠⟩⁠tiffens out its tail, and assists in drawing the rectum backwards.

I quite fail to understand the explanation of the scratching action of the dog given by your correspondent;³ a dog certainly brings his hind legs very forward when voiding but it could scarcely require the scratching action to bring them straight again, still less do I see what effect it could have in restoring the position of parts about ⁠⟨⁠    ⁠⟩⁠

With regard to the terrier dog with prolapsus ani I think his scratching propensities can be easily explained not from a desire to restore the gut, but from the sense of irritation the prolapsed gut would cause resembling that of fæces being or having been pressing on the mucous membrane, the ⁠⟨⁠action⁠⟩⁠ in question having ⁠⟨⁠be⁠⟩⁠come automatic or even reflex.⁴

Hoping the above remarks may be of some service to you | Believe me | sincerely yours | Bushell Anningson