Paranal sinus

The paranal sinuses (Latin: sinus paranales), commonly known as anal sacs, are paired, sac-like diverticula of the anal canal. They are located between the internal and external anal sphincter muscles, typically at the 4 o'clock and 8 o'clock positions relative to the anus. Each sac is lined by stratified squamous epithelium and opens into the terminal part of the anal canal via a duct.

These structures are associated with modified sebaceous and apocrine glands that secrete a foul-smelling substance. This secretion accumulates in the sacs and is normally released during defecation or as a part of social or territorial behavior in certain species.

Species differences

Dogs

In dogs, the paranal sinuses are well-developed. Each sac contains abundant apocrine glands (responsible for scent production) and some sebaceous glands. They are used in scent-marking behavior. Dogs are prone to anal sac impaction, infection, or abscessation.

Cats

In cats, the anal sacs are present but smaller than in dogs. They also contain apocrine and sebaceous glands and serve a similar purpose. Clinical problems are less frequent but can occur.

Horses, ruminants, pigs

These species do not possess true paranal sinuses or anal sacs. Consequently, they do not exhibit the same scent-marking behavior seen in carnivores.

Function

The paranal sinuses serve as scent-marking glands in carnivores. The secretion, rich in volatile compounds, is species-specific and plays a role in social communication, territorial marking, and potentially defense.

Clinical significance

Diseases of the anal sacs are common in small animals, particularly dogs. These include:

• Impaction: due to failure of normal expression

• Infection or abscessation: often secondary to impaction

• Neoplasia: such as anal sac adenocarcinoma (especially in older dogs)

Manual expression, flushing, or surgical removal (anal sacculectomy) may be indicated depending on the severity and recurrence of the condition.