# Left-Right Multimodal Morphometric Comparison of Human Submandibular Glands
## 1 Introduction  
The left-right bilateral external symmetry in vertebrates is based on the default symmetric gene expression on the development of mesodermal somites [1; 2]). On the other hand, the binary asymmetry involving the internal anatomy of the thorax and abdomen (e.g., greater blood vessels, lungs, heart, digestive tract and its glands) results as a consequence of the switch deviation from the rostro-caudal axis development. The head and neck are also subjects of left-right asymmetries, mainly concerning the muscles, skull and arteries [3; 4; 5].  
The major salivary glands, being paired, arborized and on the brink between the external and internal morphology, logically pose a question on their laterality. For instance, the submandibular gland, although being smaller than the parotid gland, accounts for two-thirds of the total saliva secretion in quiescent state. This mixed seromucinous tubule-acinar organ consists of a superficial part, which winds around the posterior edge of the mylolyoid muscle and continues as a smaller, deep part; the latter being close to the sublingual gland. The excerv duct of the submandibular gland (Wharton's duct) arises from smaller affluents in the superficial portion and runs through the deep portion, making sharp genu around the said free border of the mylohyoid. Further, after leaving the gland, it courses somewhat upward and forward in the floor of the mouth, between the hyoglossus (medially) and mylohyoid muscle (laterally), and finally opens on the sublingual caruncula, separately or jointly with the major sublingual duct (Bartholin). In the vast majority of cases, the sublingual carunculae are symmetric in position [6].  
The asymmetries of the paired head and neck anatomical entities, including the salivary glands, are often ascribed to pathology [7]. For example, the submandibular glands can be affected by a number of processes, such as saliosis, sialolitiiasis, autoimmune diseases, sialadenitis, neoplasms, sialoceles and branchial cysts. In [PERSON]'s syndrome, one of the frequently posed clinical questions is whether there is a horizontal asymmetry, i.e., difference in the left-right severity and spatial involvement [8].  
The salivary glands have been analyzed on bases of their flow rates, i.e., their secretory activities, or the influences of age, sex and BMI on their size and fat fraction, as presented on CT and MRI images [9; 10; 11]. Both aspects of research should consider the left-right (a)symmetry of the glands.  
New mechanistic hypotheses have been proposed on the development of the arborized organs, based on the fingering and branching as a consequence of liquid (or air) being pushed through the output point, with an interface of viscous instability [12]. In this context, one must also wonder whether those physical laws apply symmetrically, or not.  
In a comprehensive study on a deep learning model for the delineation and segmentation of head and neck organs at risk for cancer, [PERSON] et al. [13] produced 3D binary models which, in case of a left and right submandibular gland, presented a high correlation in the test performance.  
On the other hand, the unilateral aplasia of the major salivary glands are rare entities, detected as incidental findings on imaging. They can occur separately, or in conjunction with the hypertrophy of the contralateral gland, or other ipsilateral major glands [14; 15].  
The common quotient of all the mentioned studies is the question about whether there is a difference in the macromorphology of the normal left and right submandibular gland, as a consequence of anatomical variability. In order to answer this question, we carried out a composite multimodal morphometric analysis of postmortem submandibular glands, including linear morphometry, planimetry and fractal properties. Therefore, the Results section of the article is subdivided into those three headers.