Chitin n, a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, the scales of fish and lissamphibians; the structure of chitin is comparable to another polysaccharide—cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal and biotechnological purposes; the English word "chitin" comes from the French word chitine, derived in 1821 from the Greek word χιτών, meaning covering. A similar word, "chiton", refers to a marine animal with a protective shell; the structure of chitin was determined by Albert Hofmann in 1929. Chitin is a modified polysaccharide; these units form covalent β--linkages. Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an acetyl amine group; this allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.

In its pure, unmodified form, chitin is translucent, pliable and quite tough. In most arthropods, however, it is modified, occurring as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and molluscs, chitin produces a much stronger composite; this composite material is much harder and stiffer than pure chitin, is tougher and less brittle than pure calcium carbonate. Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar to the stiff, light elytron of a beetle. In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various iridescent colors serving phenotypic signaling and communication for mating and foraging; the elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in biomimicry.

Scarab beetles in the genus Cyphochilus utilize chitin to form thin scales that diffusely reflect white light. These scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of nanometres, which serve to scatter light; the multiple scattering of light is thought to play a role in the unusual whiteness of the scales. In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper. Chitosan is produced commercially by deacetylation of chitin. Nanofibrils have been made using chitosan. Chitin-producing organisms like protozoa, fungi and nematodes are pathogens in other species. Humans and other mammals have chitinase-like proteins that can degrade chitin. Chitin is sensed in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells.

Keratinocytes in skin can react to chitin or chitin fragments. According to in vitro studies, chitin is sensed by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, mannose receptors; the immune response can sometimes clear the chitin and its associated organism, but sometimes the immune response is pathological and becomes an allergy. Plants have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein; the first chitin receptor was cloned in 2006. When the receptors are activated by chitin, genes related to plant defense are expressed, jasmonate hormones are activated, which in turn activate systematic defenses. Commensal fungi have ways to interact with the host immune response that as of 2016 were not well understood; some pathogens produce chitin-binding proteins. Zymoseptoria tritici is an example of a fungal pathogen. Chitin was present in the exoskeletons of Cambrian arthropods such as trilobites; the oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a scorpion encased in amber.

Chitin is a good inducer of plant defense mechanisms for controlling diseases. It has been assessed as a fertilizer that can improve overall crop yields. Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods and food emulsions. Processes to size and strengthen paper employ chitosan. How chitin interacts with the immune system of plants and animals has been an active area of research, including the identity of key receptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, mechanisms by which immune systems respond. Chitin and ch

A pitch clock is used in college baseball and Minor League Baseball to limit the amount of time a pitcher uses before he throws the ball to the hitter. This is one measure. In college baseball, the Southeastern Conference experimented with using pitch clocks in 2010. Pitchers were given twenty seconds to throw the pitch. A batter stepping out of the batter's box with less than five seconds on the clock will be assessed an additional strike. After the 2010 season, the National Collegiate Athletic Association sought to make the pitch clocks mandatory, instituted it for the 2011 college baseball season, but only for when there are no runners on base. Pitch clocks made their professional debut in the Arizona Fall League in 2014. On January 15, 2015, Major League Baseball announced it would institute a 20-second pitch clock in Minor League Baseball for Double-A and Triple-A teams during the 2015 season. Pitchers were given twenty seconds to throw the pitch, with the punishment of a ball awarded to the batter if not followed.

Along with other rule changes addressing the pace of play, the clocks contributed to a 12-minute reduction in game times at those levels between the 2014 and 2015 seasons, compared to the leagues that did not use the clock, which saw game times change from an increase of three minutes per game to a decrease in five minutes per game. Game times increased in 2016 and 2017, but were still faster than games in 2014. MLB and the MLB Players Association discussed the possibility of introducing the pitch clock at the major league level for the 2018 season. MLB opted against imposing it unilaterally, over the opposition of the MLBPA. MLB implemented a 20-second pitch clock in spring training games in 2019; the independent Atlantic League uses a 12-second pitch clock. Play clock Shot clock "Pace of Game". Major League Baseball

The original Walnut Lane Memorial Bridge was a prestressed concrete girder bridge in Philadelphia, designed by Belgian Engineer Gustave Magnel and built by the City of Philadelphia. Completed and opened to traffic in 1951, this three-span bridge carried Walnut Lane over Lincoln Drive and Monoshone Creek, it was the first major prestressed concrete beam bridge designed and built in the United States when completed. The form of the bridge was simple, it looked similar to many highway bridges carrying traffic on US highways today; the bridge deck was supported by each spanning 160 feet. These girders were prestressed by post-tensioning four wire cables embedded in the concrete. Although this type of construction had been used in Europe for quite some time, the Walnut Lane Memorial Bridge was innovative in the United States and led to the successful application of this technology in this country; the material-saving bridge cost about $700,000 to construct, about 30 percent cheaper than a regular concrete arch design.

The fascia beams of the main span exhibited longitudinal cracks in about 1957. The other girders exhibited no cracks. Through the years, the cracks in the fascia beams were monitored. However, in 1989, the Pennsylvania Depart of Transportation made the decision to replace the bridge superstructure; because of the historical significance of the structure, the decision was controversial. However, the replacement structure comprises prestressed concrete girders and is similar in appearance to the original; the new bridge was completed in 1990. A bronze plaque on the bridge's abutment reads: A second plaque reads: "Outstanding Civil Engineering Achievement / Designated May 1978" List of bridges documented by the Historic American Engineering Record in Pennsylvania Zollman, Charles C.. "Building and Rebuilding of Philadelphia's Walnut Lane Memorial Bridge. Part 1: A History of Design and Service Life," PCI Journal, V. 37, No. 3, May–June 1992, pp. 66–82. Zollman, Charles C.. "Building and Rebuilding of Philadelphia's Walnut Lane Memorial Bridge.

Part 2: Demolition and Rebuilding of the Superstructure," PCI Journal, V. 37, No. 4, July–August 1992, pp. 64–82. William H. Shank, "Historic Bridges of Pennsylvania", revised ed. pp. 3–4. Newton Copp and Andrew Zanella, "A Balancing of Forces and Moments: The Walnut Lane Bridge", Discovery and Risk: Cade Studies in Science and Technology, pp. 200–214. Historic American Engineering Record No. PA-125, "Walnut Lane Memorial Bridge" Listing at Philadelphia Architects and Buildings