Skip Nav

Register now!

What is the largest organ in the human body?

❶By definition, none of these creatures has a backbone.

Top quality anatomy videos, all for free.

30 million+
Anatomy and Physiology Songs

The teeth are conical and mostly uniform in size. The surface cells of the epidermis are modified into horny scales which create a waterproof layer.

Reptiles are unable to use their skin for respiration as do amphibians and have a more efficient respiratory system drawing air into their lungs by expanding their chest walls. The heart resembles that of the amphibian but there is a septum which more completely separates the oxygenated and deoxygenated bloodstreams. The reproductive system has evolved for internal fertilization, with a copulatory organ present in most species.

The eggs are surrounded by amniotic membranes which prevents them from drying out and are laid on land, or develop internally in some species. The bladder is small as nitrogenous waste is excreted as uric acid. Turtles are notable for their protective shells. They have an inflexible trunk encased in a horny carapace above and a plastron below. These are formed from bony plates embedded in the dermis which are overlain by horny ones and are partially fused with the ribs and spine.

The neck is long and flexible and the head and the legs can be drawn back inside the shell. Turtles are vegetarians and the typical reptile teeth have been replaced by sharp, horny plates. In aquatic species, the front legs are modified into flippers. Tuataras superficially resemble lizards but the lineages diverged in the Triassic period. There is one living species, Sphenodon punctatus. The skull has two openings fenestrae on either side and the jaw is rigidly attached to the skull.

There is one row of teeth in the lower jaw and this fits between the two rows in the upper jaw when the animal chews. The teeth are merely projections of bony material from the jaw and eventually wear down. The brain and heart are more primitive than those of other reptiles, and the lungs have a single chamber and lack bronchi. The tuatara has a well-developed parietal eye on its forehead. Lizards have skulls with only one fenestra on each side, the lower bar of bone below the second fenestra having been lost.

This results in the jaws being less rigidly attached which allows the mouth to open wider. Lizards are mostly quadrupeds, with the trunk held off the ground by short, sideways-facing legs, but a few species have no limbs and resemble snakes. Lizards have moveable eyelids, eardrums are present and some species have a central parietal eye. Snakes are closely related to lizards, having branched off from a common ancestral lineage during the Cretaceous period, and they share many of the same features.

The skeleton consists of a skull, a hyoid bone, spine and ribs though a few species retain a vestige of the pelvis and rear limbs in the form of pelvic spurs.

The bar under the second fenestra has also been lost and the jaws have extreme flexibility allowing the snake to swallow its prey whole. Snakes lack moveable eyelids, the eyes being covered by transparent "spectacle" scales. They do not have eardrums but can detect ground vibrations through the bones of their skull. Their forked tongues are used as organs of taste and smell and some species have sensory pits on their heads enabling them to locate warm-blooded prey.

Crocodilians are large, low-slung aquatic reptiles with long snouts and large numbers of teeth. The head and trunk are dorso-ventrally flattened and the tail is laterally compressed. It undulates from side to side to force the animal through the water when swimming. The tough keratinized scales provide body armour and some are fused to the skull.

The nostrils, eyes and ears are elevated above the top of the flat head enabling them to remain above the surface of the water when the animal is floating. Valves seal the nostrils and ears when it is submerged. Unlike other reptiles, crocodilians have hearts with four chambers allowing complete separation of oxygenated and deoxygenated blood. Birds are tetrapods but though their hind limbs are used for walking or hopping, their front limbs are wings covered with feathers and adapted for flight.

Birds are endothermic , have a high metabolic rate , a light skeletal system and powerful muscles. The long bones are thin, hollow and very light. Air sac extensions from the lungs occupy the centre of some bones.

The sternum is wide and usually has a keel and the caudal vertebrae are fused. There are no teeth and the narrow jaws are adapted into a horn-covered beak. The eyes are relatively large, particularly in nocturnal species such as owls.

They face forwards in predators and sideways in ducks. The feathers are outgrowths of the epidermis and are found in localized bands from where they fan out over the skin. The only cutaneous gland is the single uropygial gland near the base of the tail. This produces an oily secretion that waterproofs the feathers when the bird preens.

There are scales on the legs, feet and claws on the tips of the toes. Mammals are a diverse class of animals, mostly terrestrial but some are aquatic and others have evolved flapping or gliding flight. They mostly have four limbs but some aquatic mammals have no limbs or limbs modified into fins and the forelimbs of bats are modified into wings. The legs of most mammals are situated below the trunk, which is held well clear of the ground. The bones of mammals are well ossified and their teeth, which are usually differentiated, are coated in a layer of prismatic enamel.

Mammals have three bones in the middle ear and a cochlea in the inner ear. They are clothed in hair and their skin contains glands which secrete sweat. Some of these glands are specialized as mammary glands , producing milk to feed the young. Mammals breathe with lungs and have a muscular diaphragm separating the thorax from the abdomen which helps them draw air into the lungs.

The mammalian heart has four chambers and oxygenated and deoxygenated blood are kept entirely separate. Nitrogenous waste is excreted primarily as urea. Mammals are amniotes , and most are viviparous , giving birth to live young. The exception to this are the egg-laying monotremes , the platypus and the echidnas of Australia. Humans have the overall body plan of a mammal. Humans have a head , neck , trunk which includes the thorax and abdomen , two arms and hands , and two legs and feet.

Generally, students of certain biological sciences , paramedics , prosthetists and orthotists, physiotherapists , occupational therapists , nurses , podiatrists , and medical students learn gross anatomy and microscopic anatomy from anatomical models, skeletons, textbooks, diagrams, photographs, lectures and tutorials, and in addition, medical students generally also learn gross anatomy through practical experience of dissection and inspection of cadavers.

The study of microscopic anatomy or histology can be aided by practical experience examining histological preparations or slides under a microscope. Human anatomy, physiology and biochemistry are complementary basic medical sciences, which are generally taught to medical students in their first year at medical school. Human anatomy can be taught regionally or systemically; that is, respectively, studying anatomy by bodily regions such as the head and chest, or studying by specific systems, such as the nervous or respiratory systems.

Academic anatomists are usually employed by universities, medical schools or teaching hospitals. They are often involved in teaching anatomy, and research into certain systems, organs, tissues or cells. Invertebrates constitute a vast array of living organisms ranging from the simplest unicellular eukaryotes such as Paramecium to such complex multicellular animals as the octopus , lobster and dragonfly.

By definition, none of these creatures has a backbone. The cells of single-cell protozoans have the same basic structure as those of multicellular animals but some parts are specialized into the equivalent of tissues and organs. Locomotion is often provided by cilia or flagella or may proceed via the advance of pseudopodia , food may be gathered by phagocytosis , energy needs may be supplied by photosynthesis and the cell may be supported by an endoskeleton or an exoskeleton.

Some protozoans can form multicellular colonies. Metazoans are multicellular organism, different groups of cells of which have separate functions. The most basic types of metazoan tissues are epithelium and connective tissue, both of which are present in nearly all invertebrates. The outer surface of the epidermis is normally formed of epithelial cells and secretes an extracellular matrix which provides support to the organism.

An endoskeleton derived from the mesoderm is present in echinoderms , sponges and some cephalopods. Exoskeletons are derived from the epidermis and is composed of chitin in arthropods insects, spiders, ticks, shrimps, crabs, lobsters.

Calcium carbonate constitutes the shells of molluscs , brachiopods and some tube-building polychaete worms and silica forms the exoskeleton of the microscopic diatoms and radiolaria. The outer epithelial layer may include cells of several types including sensory cells, gland cells and stinging cells. There may also be protrusions such as microvilli , cilia, bristles, spines and tubercles. Marcello Malpighi , the father of microscopical anatomy, discovered that plants had tubules similar to those he saw in insects like the silk worm.

He observed that when a ring-like portion of bark was removed on a trunk a swelling occurred in the tissues above the ring, and he unmistakably interpreted this as growth stimulated by food coming down from the leaves, and being captured above the ring. Arthropods comprise the largest phylum in the animal kingdom with over a million known invertebrate species. Insects possess segmented bodies supported by a hard-jointed outer covering, the exoskeleton , made mostly of chitin.

The segments of the body are organized into three distinct parts, a head, a thorax and an abdomen. The thorax has three pairs of segmented legs , one pair each for the three segments that compose the thorax and one or two pairs of wings. The abdomen is composed of eleven segments, some of which may be fused and houses the digestive , respiratory , excretory and reproductive systems.

Spiders a class of arachnids have four pairs of legs; a body of two segments—a cephalothorax and an abdomen. Spiders have no wings and no antennae.

They have mouthparts called chelicerae which are often connected to venom glands as most spiders are venomous. They have a second pair of appendages called pedipalps attached to the cephalothorax. These have similar segmentation to the legs and function as taste and smell organs. At the end of each male pedipalp is a spoon-shaped cymbium that acts to support the copulatory organ. In BCE, the Edwin Smith Papyrus , an Ancient Egyptian medical text , described the heart , its vessels, liver , spleen , kidneys , hypothalamus , uterus and bladder , and showed the blood vessels diverging from the heart.

The Ebers Papyrus c. Ancient Greek anatomy and physiology underwent great changes and advances throughout the early medieval world. Over time, this medical practice expanded by a continually developing understanding of the functions of organs and structures in the body.

Phenomenal anatomical observations of the human body were made, which have contributed towards the understanding of the brain, eye, liver, reproductive organs and the nervous system. The Hellenistic Egyptian city of Alexandria was the stepping-stone for Greek anatomy and physiology. Alexandria not only housed the biggest library for medical records and books of the liberal arts in the world during the time of the Greeks, but was also home to many medical practitioners and philosophers.

Great patronage of the arts and sciences from the Ptolemy rulers helped raise Alexandria up, further rivalling the cultural and scientific achievements of other Greek states. Some of the most striking advances in early anatomy and physiology took place in Hellenistic Alexandria.

These two physicians helped pioneer human dissection for medical research. They also conducted vivisections on the cadavers of condemned criminals, which was considered taboo until the Renaissance — Herophilus was recognized as the first person to perform systematic dissections. He was able to distinguish the sensory and the motor nerves in the human body and believed that air entered the lungs and heart, which was then carried throughout the body.

His distinction between the arteries and veins — the arteries carrying the air through the body, while the veins carried the blood from the heart was a great anatomical discovery. Erasistratus was also responsible for naming and describing the function of the epiglottis and the valves of the heart, including the tricuspid. Great feats were made during the third century in both the digestive and reproductive systems.

Herophilus was able to discover and describe not only the salivary glands, but the small intestine and liver. He recognized that spermatozoa were produced by the testes and was the first to identify the prostate gland.

The anatomy of the muscles and skeleton is described in the Hippocratic Corpus , an Ancient Greek medical work written by unknown authors. Praxagoras identified the difference between arteries and veins. Also in the 4th century BCE, Herophilos and Erasistratus produced more accurate anatomical descriptions based on vivisection of criminals in Alexandria during the Ptolemaic dynasty.

In the 2nd century, Galen of Pergamum , an anatomist , clinician , writer and philosopher , [67] wrote the final and highly influential anatomy treatise of ancient times. Anatomy developed little from classical times until the sixteenth century; as the historian Marie Boas writes, "Progress in anatomy before the sixteenth century is as mysteriously slow as its development after is startlingly rapid". It was the standard anatomy textbook for the next century.

Leonardo da Vinci — was trained in anatomy by Andrea del Verrocchio. Andreas Vesalius — Latinized from Andries van Wezel , professor of anatomy at the University of Padua , is considered the founder of modern human anatomy. In England, anatomy was the subject of the first public lectures given in any science; these were given by the Company of Barbers and Surgeons in the 16th century, joined in by the Lumleian lectures in surgery at the Royal College of Physicians.

In the United States, medical schools began to be set up towards the end of the 18th century. Classes in anatomy needed a continual stream of cadavers for dissection and these were difficult to obtain. Philadelphia, Baltimore and New York were all renowned for body snatching activity as criminals raided graveyards at night, removing newly buried corpses from their coffins.

The practice was halted in Britain by the Anatomy Act of , [81] [82] while in the United States, similar legislation was enacted after the physician William S. Forbes of Jefferson Medical College was found guilty in of "complicity with resurrectionists in the despoliation of graves in Lebanon Cemetery". He was responsible for setting up the system of three years of "pre-clinical" academic teaching in the sciences underlying medicine, including especially anatomy. This system lasted until the reform of medical training in and As well as teaching, he collected many vertebrate skeletons for his museum of comparative anatomy , published over 70 research papers, and became famous for his public dissection of the Tay Whale.

He noticed that the frequently fatal fever occurred more often in mothers examined by medical students than by midwives. The students went from the dissecting room to the hospital ward and examined women in childbirth. Semmelweis showed that when the trainees washed their hands in chlorinated lime before each clinical examination, the incidence of puerperal fever among the mothers could be reduced dramatically.

Before the modern medical era, the main means for studying the internal structures of the body were dissection of the dead and inspection , palpation and auscultation of the living. It was the advent of microscopy that opened up an understanding of the building blocks that constituted living tissues. Technical advances in the development of achromatic lenses increased the resolving power of the microscope and around , Matthias Jakob Schleiden and Theodor Schwann identified that cells were the fundamental unit of organization of all living things.

Study of small structures involved passing light through them and the microtome was invented to provide sufficiently thin slices of tissue to examine. Staining techniques using artificial dyes were established to help distinguish between different types of tissue. Advances in the fields of histology and cytology began in the late 19th century [89] along with advances in surgical techniques allowing for the painless and safe removal of biopsy specimens.

The invention of the electron microscope brought a great advance in resolution power and allowed research into the ultrastructure of cells and the organelles and other structures within them. About the same time, in the s, the use of X-ray diffraction for studying the crystal structures of proteins, nucleic acids and other biological molecules gave rise to a new field of molecular anatomy.

Equally important advances have occurred in non-invasive techniques for examining the interior structures of the body. X-rays can be passed through the body and used in medical radiography and fluoroscopy to differentiate interior structures that have varying degrees of opaqueness. Magnetic resonance imaging , computed tomography , and ultrasound imaging have all enabled examination of internal structures in unprecedented detail to a degree far beyond the imagination of earlier generations.

From Wikipedia, the free encyclopedia. For the anatomy of plants, see Plant anatomy. For other uses, see Anatomy disambiguation. Arthropod , Insect morphology , and Spider anatomy. History of anatomy in the 19th century. Archived from the original on 16 March Retrieved 19 March Relevance of human anatomy in daily clinical practice. Annals of Anatomy-Anatomischer Anzeiger, 6 , — Retrieved 8 July Journal of Crustacean Biology.

Journal of Zoo and Wildlife Medicine. Retrieved 29 April Invertebrate Zoology, 7th edition. Retrieved 25 June B; Whillis, J, eds. Descriptive and Applied 28 ed.

Nervous System 4th ed. Retrieved 30 April Retrieved 13 July Functional anatomy of the vertebrates: National Center for Science Education. Retrieved 28 June Archived from the original on 28 April I am taking a 12 week summer course over Anatomy and Physiology 1. I have heard it is the hardest pre req we have to take and on top of that i went to rate my professor and apparently she is a horrible teacher!

I feel like I am going to have to teach myself! Please and Thank you. I would have struggled without my mentor and the long hours I spent in the lab studying models. Here are some websites that I love so much I still refer back to them: Human Body Maps take some time to go through this. I LOVE this site! A list of valuable resources. Find which ones you like and will work for you: Medical Matrix - Anatomy 5. Visible Body I listed this last because you have to pay for it.

I have it on my iphone and love it. But if you are just starting out, it can be a bit overwhelming Visible Body 3D Human Anatomy If you can use a lab manual for anatomy then you might want to try that. Since you have an online class, I would find a cheap one-- not the newest version.

Writing things out always helped me like tracing a drop of blood. Some people say get a coloring book. Honestly that never appealed to me.

Navigation menu

Main Topics

Privacy Policy

A free website study guide review that uses interactive animations to help you learn online about anatomy and physiology, human anatomy, and the human body systems. Start Learning now!

Privacy FAQs

Need homework and test-taking help in Anatomy and Physiology? These articles can help you understand the Anatomy and Physiology of the human body.

About Our Ads is a free virtual human anatomy website with detailed models of all human body systems. The Internet's best anatomy learning resource! We have the most comprehensive range of 3D anatomy tutorials on the internet, covering all topics of anatomy. We are constantly adding to and improving our wide range of tutorials to help you revise anatomy!

Cookie Info

Get introduced to the major organ systems of the human body! You’ll learn some general anatomy (a roadmap of your body), learn how the arm bone actually connects to the shoulder bone, and how the different organs work together to keep you alive. I am taking a 12 week summer course over Anatomy and Physiology 1. I have heard it is the hardest pre req we have to take and on top of that i went to rate my professor and apparently she is a.