The lacrimal canaliculus is divided into the vertical and horizontal portions [1, 2, 6]. Its transitional part occasionally dilates to form an irregular dilated cavity or ampulla (Fig. 3.2) [6]. The length of the vertical portion is 2 mm and that of the horizontal part is 10 mm [6]. The medial most 2 mm of the horizontal portion mostly forms the common duct or canaliculus [5, 9], and more than half of this part runs in the wall of the sac. The punctum and vertical canaliculus are encircled by a similar hard fibrous tissue. This fibrous tissue in the vertical canaliculus contains skeletal muscle fibers, called the muscle of Riolan (Fig. 3.1) [10]. The epithelium of the canaliculus is a nonkeratinized stratified squamous epithelium, similar to the punctum (Fig. 3.3) [10]. As the canalicular wall contains much elastic fibers (Fig. 3.4), its diameter can be changed to enlarge or shrink as needed. Although the diameter of the canaliculus is usually 0.3–0.6 mm [1, 8], it can be expanded to over 1.0 mm. The temporal 4/5 part is directed posteronasally and surrounded by the Horner’s muscle, occasionally called the lacrimal part of the orbicularis oculi muscle (Fig. 3.5) [8]. In the nasal 1/5 part, the Horner’s muscle directs posteriorly away from the canaliculus (Fig. 3.5) [8]. Although the canaliculus usually directs anteronasally after separation from the Horner’s muscle (Fig. 3.5), it occasionally directs posteronasally in cases with proptosis (Fig. 3.6) [8]. The superior canaliculus courses, in general, almost straight to the internal common ostium, but the inferior canaliculus changes the course superiorly before joining the superior canaliculus. The course of the inferior canaliculus to be independently emptying into the sac has not been known so far.

More than 95 % of the upper and lower canaliculi join to become the common canaliculus to reach the common internal ostium [5, 11]. The canaliculi empty into the sinus of Maier (Fig. 3.7) and those independently pouring into the sac are <2 % [11]. Sinus of Maier needs further elaboration. The common internal ostium is the part where the common canaliculus pours to the sac. However, the common canalicular cavity does not simply connect with the sac lumen. A laterally bulged portion of the sac, called the sinus of Maier, is occasionally formed around the common internal ostium and some canaliculi empty to this portion (Fig. 3.7) [11]. An expanded common canaliculus can also be called the sinus of Maier (Fig. 3.4) [11].

The common canaliculus has a nonkeratinized stratified squamous epithelium. However, the transitional area with the stratified columnar epithelium of the sac with some goblet cells is occasionally seen in the common canaliculus (Fig. 3.3) [8]. To the contrary, the stratified squamous epithelium of the common canaliculus sometimes extends to the sac lumen (Fig. 3.8) [8].

A protuberance (fold) is shown, although only a half of cases [9], at the junction between the common canaliculus and the sac [12]. This structure is called the valve of Rosenmüller [5, 6]. The common internal ostium largely opens by temporal traction of the Horner’s muscle during eye closing, but there is a nasal movement of the ostium as well [13]. Therefore, the part around the common internal ostium needs a structure dealing with this nasal movement and this may be the real reason for a valvular presence in this region. The sinus of Maier could have also been evolved for the same reason.

The lacrimal sac and the nasolacrimal duct are a continuous structure [14]. The part within the lacrimal sac fossa is called as the “sac,” and the part inferior to the superior opening of the nasolacrimal canal is the “nasolacrimal duct.” [14] The part of the sac superior to the medial canthal tendon (MCT) is called the fundus, with its vertical length of 3–5 mm [14]. The body of the sac, inferior to the MCT, is about 10 mm in length. The epithelium of the sac is a stratified columnar epithelium (Figs. 3.3, 3.6, and 3.8) [15] and contains goblet cells, cilia, and serous glands [16]. The epithelial surface shows microvilli [17, 18]. Although the sac wall is constituted with a cavernous structure, it is fairly thin and less developed than that of the nasolacrimal duct [16, 19]. The lateral aspect of the sac wall is covered by a fascia, and its posterior portion is a common fascia with the Horner’s muscle (Fig. 3.6), which is called the “lacrimal diaphragm” [14].

The lacrimal sac fossa comprises of the anterior frontal process of the maxillary bone and the posterior lacrimal bone [9]. There are ridges anteriorly and posteriorly, which are called the anterior or posterior lacrimal crest, respectively (Fig. 3.9) [5]. The suture between the maxilla and the lacrimal bone is situated in various ways, and some take its site close to the posterior lacrimal crest. A process is formed between the inferior portion of the posterior lacrimal crest and the orbital face of the maxilla, which is called the hamular process (Fig. 3.10) [6]. A groove is shown nasal to the anterior lacrimal crest which is called the sutura notha [5], sutura longitudinalis imperfecta [6], or pseudo-suture [20]. It is not considered as a true suture but a vessel groove formed by a branch of the inferior orbital artery [9].

The superoinferior length of the lacrimal sac fossa is 12–15 mm, anteroposterior 4–9 mm, and the width 2–3 mm (Figs. 3.9 and 3.10) [9]. The lacrimal sac fossa shows shorter anteroposterior length superiorly [21, 22]. As the lacrimal sac fossa opens temporally, the sac lumen is usually large enough. The long axis of the fossa inclines about 10° poteriorly [23] (Fig. 3.11) and directs about 10° temporally [24] (Fig. 3.12). The angle range of the long axis of the fossa is 0–20° posteriorly [23] and 1–30° temporally [24].

The lacrimal sac and the nasolacrimal duct are a continuous tissue, and anatomically speaking, the “nasolacrimal duct” (mucosal portion) is the part inferior to the superior opening of the nasolacrimal canal (bony portion) [14]. The nasolacrimal canal is formed by the lacrimal bone superonasally, the inferior turbinate bone inferonasally, and the maxillary bone temporally [5]. The superior opening is about 6 mm in diameter and, in general, is an ellipse with a little longer horizontally (Fig. 3.16). The supero-inferior length of the canal is fairly short, about 12 mm [14] (Fig. 3.12). Although the longitudinal axis of the canal directs about 20° posteriorly [23] (Fig. 3.11), it directs almost vertically in most cases [24]. The nasolacrimal canal empties into the superior part of the inferior meatus (Fig. 3.12).

The angle range of the long axis of the nasolacrimal canal is 3–40° posteriorly [23]. The frontal view shows the angle range from 12 ° nasally to 11° temporally and mostly directs vertically around 0° [24]. Although a general consensus of the canal course is “temporal,” occasionally cases with medial course have been noted.

The nasolacrimal canal does not have a constant diameter throughout its length: some shows narrower and others larger [27]. Two thirds to 3/4 of cases show the narrowest part at the superior opening, but the others have found the narrowest portion at 3.5–5.5 mm from the superior opening [27]. These narrowings may have a bearing on the etiopathogenesis of primary acquired nasolacrimal duct obstructions (PANDO) [27].

Epithelium of the NLD is a stratified columnar epithelium, similar to the lacrimal sac, and contains goblet cells and serous glands [15]. In general, the goblet cells are distributed more inferiorly, but several specimens have also demonstrated considerable number of goblet cells throughout (Figs. 3.17, 3.18, and 3.19). Although the cavernous structure is shown similar to the lacrimal sac, it is much more developed than the sac [16, 19] (Figs. 3.17 and 3.18). The wall is more thickened inferiorly and most show a funnel shape lumen (Fig. 3.20). Cilia are similar to the nasal mucosal cells [18]. The microvilli on the epithelial surface contribute to reabsorption of the lacrimal fluid [17, 18].