Purpose: To examine Müller's muscle's horizontal extensions in relation to the peribulbar smooth muscle network. Design: Observational anatomic study. Participants: Twenty postmortem orbits (10 right, 10 left) of 15 Asians (8 males, 7 females; age range at death, 6193 years; mean age, 78.4 years) fixed in 10% buffered formalin. Methods: After performing a full-thickness 360° incision of the periosteum around the circumference of the orbit, the periosteum was elevated and finally detached near to the orbital apex. Nerves, blood vessels, and the nasolacrimal duct arising from the orbital wall were cut. The lateral orbital wall then was removed at approximately 3 cm posterior to the orbital rim and the retrobulbar content was incised with a sharp scalpel in a coronal plane. The removed orbital content was incised at a plane passing from a point located 15 mm superior to the upper eyelid margin and the globe equator at 3- and 9-o'clock areas. The sliced specimens were dehydrated and embedded in paraffin, cut into 7-μm thickness sections, and then stained with Masson trichrome. Main Outcome Measures: The medial and lateral extensions of Müller's muscle in relation to the peribulbar smooth muscle network. Results: In all specimens, Müller's muscle extended medially and laterally. The medial extension reached the medial rectus muscle pulley, which is rich in smooth muscle fibers. The lateral extension reached the lateral rectus muscle pulley by passing through the lacrimal gland fascia of the palpebral lobe, in which 12 specimens also showed a direct extension to the lateral rectus muscle pulley in the posterior part. Conclusions: Müller's muscle has a medial and a lateral extension to the peribulbar smooth muscle network. These new findings indicated that Müller's muscle is not an independent structure in the upper eyelid, but rather a component of the peribulbar smooth muscle network. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.