Crying is the process in human or response to an emotional state that is characterized by the shedding tears from the from the lacrimal apparatus of the eye. A young boy falls and scrapes his knee, his body sends signals to his brain telling him that something has gone wrong. His brain sends something back to his body; pain. Tears fall down his face and he lets out a wail.
Most people have gone through this experience of crying. You may have also cried after watching a sad movie, getting your feelings hurt, experiencing physical pain, or just having something irritate your eye. Crying is a universally understood phenomenon. Most people understand that humans cry out of pain, hurt, or even joy. It may seem strange to some that our tear ducts decide to overload just at these moments, but our Creator has made this system extremely complex because it serves a larger purpose than just to lubricate our eyes. Tears allow us to fight infection, depression, and show emotion in a way that no other beast on our planet can. Tears make us distinctly human and show the complexity and ingenuity of our God.
Many people would say our eyes create tears, however this is not entirely true. Our body provides tears through glands above each eye, behind the upper eyelids. These lacrimal glands secrete lacrimal fluid, or what most people know as tears. Lacrimal fluid is normally very salty, but can vary due to the cause of the tears. When the brain signals for the glands to secrete tears, gravity or capillary action draws the lacrimal fluid onto the eye’s surface. These tears will either be used to lubricate the eye, wash away a fragment, or collect on the lower eyelid. The excess tears will usually collect in a place named the lacrimal lake where they will be drawn to the lacrimal duct. If there is too much fluid to drain, or if the duct is blocked, extra tears may spill out of the eye and onto the face. If the tears do go through the duct they may be ingested or come out of the nasal passages. 
There are three types of tears. Basal tears are continuous and are used to lubricate our eyes. Reflex/Infant tears occur in instances such as when we slice onions or when something hits our eye. The last group, Emotional tears, is psychologically created. It has been shown that emotional tears and irritant tears are actually composed of different substances. Emotional tears, for example contain a protein concentration that is 24% stronger than in irritant tears. The proteins discovered in the lacrimal fluid were found to be used in the human stress response. It has been considered that due to these findings, tears can actually expel toxins from the body. It has also been shown that after sexual arousal, the body often uses crying as a way to rebound from such sympathetic nervous system hyperactivity. Emotional tears have also been proven to hold higher levels of magnese and prolactin. The reduction of both of these substances in the body help keep depression at a more suppressed level. Some people find that after being upset, a cry helps calm them due to the hormones and chemicals which are released in the lacrimal fluid.
Many people today believe that crying is not only beneficial to our health, but also essential in maintaining our health. A man by the name of Randolph Cornelius compiled and analyzed a group of articles ranging from 1850 to 1989 on the topic of crying. 94% of the articles recommended crying. They had realized that crying was a major part of releasing physiological tensions. If the person did not let the feelings out emotionally, they would find a different way to be released; such as affecting the body by disease. Psychotherapist and counselor alike, from nearly every school or persuasion, believe crying in the process of therapy as helpful, although exceptions such as depression, neuropsychological conditions, or manipulative tears are not regarded as such. Some people may enter a therapy office for a beginning session and cry within minutes out of the relief of being in the presence of a therapist, an unconnected stranger. Patients who cry during sessions during a point of expressing subjects of important emotional importance to them usually say they feel much better afterwards. In a normal situation, people try to persuade others not to cry, while in a psychologist’s office, the person is directed in a way that is more likely to cause tears. 
Besides the semi-automatic production of tears due to lubrication or irritant-eliminating needs, a person can also generate tears through strong emotions, even by using ‘sense memory’. Trained actors can often create a genuine emotional response during a dramatic scene by channeling old memories and emotions. These are often called “crocodile tears” because they are not actually crying about what is going on in the scene exactly, but crying based off of their own experiences. Hard squinting can also cause tears to pool up.
A man by the name of Cornelius researched in 2001 whether the belief of tears being beneficial was correct. A test was run in over 29 nations. It showed that when a person is asked personal questions, such as through interview, questionnaire, or diary keeping they usually felt happier, relieved, or more relaxed after crying about the subjects. However if people were affected by something non-personal, such as a sad movie, they either did not benefit much or felt even worse afterwards. This form of laboratory testing- using non-personal subjects to stimulate crying- has found crying ineffective or harmful. However the use of sad, non-personal ideas in a controlled environment, not necessarily a ‘real-life’ scenario may be creating incorrect results. It has also been mentioned that possibly crying is not used to release or rebound from arousal, but actually causes it. The contrast in arousal from crying to being calm may also bring the illusion that things are better, when in reality, the person may be just as sad as before they had cried. In general however, two parts of crying have been found beneficial: the physical act of crying and also understanding and investigating the issue during crying.  One woman who experienced migranes found that emotional tears, and not ones made out of joy or by activities such as cutting onions, triggered her migrane symptoms. It is not common for this to occur but it can be an effect of crying.
The nasolacrimal duct is a duct that allows tears to flow from the eyes to the back of the nose. Two tiny holes, located in the corner of the eye close to the nose, are connected to the nasolacrimal duct, which travels through a canal, named the nasolacrimal canal, to the back of the nose. This is why when a person cries, their nose will also run. This is the overflow of tears to the back of the nose. In a normal circumstance, the eye is covered by a layer of tears, which are swept away by the nasolacrimal duct to maintain an appropriate amount of fluid. This correct amount reduces infection. When someone touches their eye, it can cause an infection because the water is drained to the back of the nose, throat, and throughout the body. This is why people should avoid touching their eyes or thoroughly wash their hands before touching their eyes or eyelids. 
The epithelial lining of the lacrimal sac and the nasolacrimal duct is faced by microvilli, cellular membrane protrusions which increase the area of the nasolacrimal duct. Animal experiments with rabbits have supported the thought that tear fluid components can and are absorbed in the human nasolacrimal system. Evidence of seromucous glands, glands which secrete substances not related to their ordinary metabolic needs, intraepithelial lipids and a specific mucus layer show that the normal three-layered tear film is possibly present in the nasolacrimal ducts. The mucus layer, which is composed mainly of goblet cells and intraepithelial mucus glands, serves as a way to help drain lacrimal fluid. This mucous layer has been considered similar to the epithelia of the gastrointestinal tract. There are also defense mechanisms in the ducts. Antimicrobial peptides like IgA and immunocompetent cells (lymphocytes and macrophages), slow intraepithelially and subepithelially circulation.
The wall of the lacrimal sac and the nasolacrimal duct consist of a spiral-like system of connective tissue fibers. Wide inner cavity vascular networks are located in the system. Towards the bottom the vascular system is connected to the inferior turbinate, an extension of the ethmoid bone in the nose. With swelling the system may have the tears removed due to its centered attachment and spirally arranged fiber structures. This way the lacrimal fluid is drained away from the ducts and out of the eyes. The blood vessels are supported by their growth. They regulate blood flow by opening and closing the cavity of the lacrimal passage due to the level of lacrimal fluid in the passage way, which regulates tear flow. However, during epiphoria caused by an emotional response, this process is different, not regulating the flow of tears as regularly done. However, malfunctions in the cavernous body or in the blood vessels located in that area, may disturb tear outflow. Eye congestion or even occulation, may cause the passage way in the lacrimal passage to close.
Malfunctions in the nasolacrimal duct or in the blood vessels located in that area, may disturb tear outflow causing eye congestion or even occulation, closing of the passage way in the lacrimal passage, may occur. 
A study of axial cut maxillofacial tomographic scans showed that men and women have different size nasolacrimal ducts. The three levels of bony nasolacrimal duct (NLD) system on both the left and right sides of the face were measured the diameter from front to back. 36 men and 35 women were studied. Women showed to have a smaller bony diameter at the lower nasal cavity and the middle cavity. It showed that, with age, the lower nasal cavity increased in males and females, while the middle NLD only increased in men. The right and left sides stayed at around the same size and one side did not grow more than the other. Due to women having smaller lower lacrimal cavity space and a lack of growth in the middle canal, women are more likely to suffer from NLD obstruction.
In certain people, the nasolacrimal duct can become blocked by an irritant or become infected and become inflammed. This causes epiphoria, a situation in which tears flow out of the eye instead of traveling through the duct. This problem often occurs in people with chronic allergies. This explains why those with hay fever and environmental allergies often have runny eyes. If this blocking continues for a long period of time the eyes may secrete a pus-like discharge. By removing the allergen or taking medication to reduce inflammation the blockage can be removed. 
The study of disorders or diseases which can cause dacryostenosis, obstruction or narrowing of the nasolacrimal duct, includes learning of downward inflammation from the eye or rising inflammation from the nose causing a swelling in the mucus membrane. This rearranges the spiral-like arrangement of the connective tissue fibers and causes malfunctions in the cavernous body below the epithelium causing blood to enter the tissue and cause temporary blockage of the lacrimal passage.
A dacryocystorhinostomy operation can be made if a patient experiences epiphora (overflow of tears onto the face). This operation involves creating a new passageway for tears to drain into the nasal passage. This operation may be external or endoscopic. The endoscopic option has many advantages including:
- It provides better aesthetic results and there is no external scarring.
- It is a one-stage procedure which can also correct associated nasal diseases or disorders.
- It avoids injury to the medial canthus, an opening to the lacrimal sac.
- It preserves the orbicularis oculi muscles pumpng mechanism
- Surgery does not cause active infection.
- If revision is needed, it is much less invasive or difficult.
- It causes less if any blood loss, unlike the external approach
- It take much less time to complete
- The success rate is comparable to the external approach
There are also some disadvantages:
- It requires a specialist in nasal endoscopic surgery
- Nasal endoscopic surgery equipment causes extra expense
The external approach to dacryocystorhinostomy was first recorded by Toti in 1904 and then the endoscopic by West in 1911. The endoscopic was not very popular due to a lack of tools to see the lacrimal sac. However with rigid telescopes, these problems have been removed and the endoscopic technique is now becoming more popular.
Endoscopic tear duct surgery, dacryocystorhinostomy, DCR, ostium by Dr. Korn. He is using flourescein eye drops like the ones used in Schirmer's test. See top diagram.
Here is a clearer view of the location of the Nasolacrimal Duct and possible procedures which can be done to this system if a problem should occur.
- How do Our Eyes Create Tears? by Michael Pollick, wiseGEEK. 12 April 2010.
- Tears - nature's emotional processing? by Cornelia House. Accessed on:03 May 2010.
- Why we Laugh and Cry by Wendy Norlund.gibbsmagazine Accessed, May 9, 2010.
- Tears and Processing of Emotional Hurt by Cornelia House. Accessed, May 8, 2010.
- Nasolacrimal Duct :: anatomy & histology by S A Groessl, B S Sires, B N Lemke Department of Ophthalmology and Visual Sciences, University of Wisconsin, Madison, USA.. October 1992.
- Dacryocystorhinostomy by S Valentine Fernandes, MBBS, MCPS, FRCSEd, FRACS, FACS, LLB, Conjoint Senior Clinical Lecturer, Department of Otorhinolaryngology, Newcastle University; Senior Consultant Surgeon, Department of Otorhinolaryngology-Head and Neck Surgery, John Hunter, Warners Bay Private Hospitals, Australia. Updated: Apr 26, 2010.