With the development of society and the sustenance of human emotions, more and more families or individuals are eager to get companionship and choose to keep pets. Cats, dogs, rabbits, and other animals are mostly furry, providing "contact comfort" for humans, allowing travelers to relieve loneliness and homesickness, enabling the elderly left behind to have expectations and dependence, and providing emotional support for workers under pressure and adding joy to their lives.

Cats, belonging to Felidae family, are divided into domestic cats and wild cats. The cat has a cute appearance, and its ability to act coquettishly and cutely is impressive. However, this is not the nature of cats. It should be noted that tigers and lions also belong to Felidae family, and they may have a common ancestor with cats (Bai, 2017). At present, there are over 100 types of cats in the world, with over 30 commonly found and over 40 certified by the International Cat Federation in the United States. Different types of cats have different breeds, and even the same breed can have multiple breeds. But the basic appearance is: round head, short facial area, five fingers on the forelimbs, four toes on the hind limbs, with sharp and curved claws at the toe tips that can retract.

The body of a cat is divided into five parts: head, neck, trunk, limbs, and tail. This review explores the physiological characteristics of cats, from the digestive system, circulatory system, respiratory system, reproductive system, central nervous system, bones, cat claws, to the extremely sensitive senses that facilitate hunting: vision, hearing, smell, taste, and touch. By exploring the physiological characteristics of cats, cat owners can gain a deeper understanding of their body structure.

1 The Origin and Development of Cats

1.1 The origin of cats

There are records of cats in ancient Egypt as early as 3 500 years ago. From the murals of temples along the Nile River, it can be seen that ancient Egypt had a tradition of raising domestic cats. There have been records in history of cats catching mice, and with the development of history, cats gradually became deified and became the gods of ancient Egyptians, worshipped by local people. Legend has it that the ancient Egyptian goddess Best was the lion headed human body (also known as the cat headed human body) (Figure 1). Every night, she transformed into a cat with extraordinary strength and speed. So the pharaohs of ancient Egypt liked to call themselves "the son of Best", which shows that the status of cats in the hearts of ancient Egypt was very noble.

The ancestor of Asian cats is the "Indian Desert Cat", which first appeared in the Indus Valley region over 2 000 years ago. However, cats appeared relatively late in China. 猫 (Mao, cat) is a pictophonetic character in Chinese, with 犬 (Quan) on the left and 苗 (Miao) on the right. In Shuowen Jiezi (Origin of Chinese Characters), it is written that Quan refers to animals, and most Chinese characters of animals have a reversed Quan cahracter. The Miao character has grass above and a field below, indicating small grass in the field. Here, the extended meaning "small" is used, and "Miao" is also the sound of a cat's call. Putting these Chinese characters together, character 猫 (Mao) is formed, finally, originally referring to small beasts in the field. In ancient China, farmers were often affected by rodent infestations when farming, resulting in minimal harvests. Later, they accidentally discovered an animal called the civet, also known as the wild cat. People have found that beavers can easily capture mice, and in order to increase harvests and reduce rodent damage, people have begun to domesticate beavers. In order to distinguish between domestic and wild animals, the word 'cat' was created separately. This is the origin of Chinese cat and its character.

1.2 The evolution of cats

The evolution of cats can be traced back to the southern European region, starting about 90 000 years ago. Cats are arthropods, most of which are herbivores. Their development has gradually formed a stronger carnivore animal - "cat", which has become today's domestic cat (Figure 2).

Carnivorous animals evolved during the Neogene Paleocene, serving as the common evolutionary ancestor of all modern carnivores. Carnivorous animals have long bodies, short limbs, clawed feet, 44 teeth, and underdeveloped brains. In the Eocene, the sarcoidodontes weakened, and the smaller archaeopteryx, the evolutionary species, replaced them. The little ancient cat is the evolutionary ancestor of all modern terrestrial carnivores, inhabiting forests and having a more developed brain than carnivores, resulting in higher predation efficiency.

During the Oligocene period, the ancient carnivores branched into the appearance similar to modern carnivores, such as Dinictis. The body shape of Dinictis is similar to that of a civet and a cat, with long limbs and tail, and it walks on the ground using the soles of its feet. In the Miocene, a group of Pseudoelurus was developed. The Pseudoelurus had a very cat-like body and began to walk on the ground with their toes. The arrangement of teeth was also the same as that of modern cats. During the Pliocene, Pseudoelurus evolved into the Felis lunensis, which is a true modern Felidae family, same as modern cats in terms of size and body shape. Modern cats evolved in the Pleistocene (Driscoll et al., 2007).

1.3 Domestication of cats

Cat is a domesticated pet that had a history of human domestication long before the emergence of ancient Egyptian civilization. After thousands of years of domestication, humans have softened this animal and make them kind, enabling cats to coexist harmoniously with humans, and continuously adjusting and optimizing the appearance, body shape, and behavioral characteristics of various breeds.

In ancient ruins dating back over 10 000 years, archaeologists have discovered many fossils of small Felines, indicating that cats have entered human life since then (Wu, 2022). In the early 21st century, archaeologists excavated cat bones that were adjacent to human bones in a 9 500 year old tomb in Cyprus. Based on the results of this archaeological discovery, combined with genetic research on cats, scientists estimate that the domestication of cats began approximately 10 000 years ago in the "crescent fertile soil". At that time, wild cats were tamed into domestic cats by humans, and later, domestic cats were brought from here to places such as Egypt and Cyprus.

However, China domesticated cats relatively late, and the earliest domestication of cats was recorded in Han Feizi Yang Quan Pian of the Warring States period's, which stated that "the rooster was responsible for reporting the arrival of the morning, and the leopard cat was responsible for catching the mouse, all using their abilities" (Figure 3). This is the earliest documented book on taming cats that can be found in literature. China raised cats on a large scale during the Sui and Tang dynasties.

2 Physiological Characteristics of Cats

2.1 Digestive system

The digestive system of cats consists of two major parts: the digestive tract and digestive glands (Figure 4). The digestive tract includes the mouth, throat, esophagus, stomach, small intestine, large intestine, and anus. The digestive glands include 5 pairs of sleeping fluid glands (ear gland, collar gland, sublingual gland, molar gland, and orbital gland), liver, pancreas, gastric spring, and intestinal gland.

The oral cavity of cats can be divided into vestibular, buccal, and oral regions. In adult cats, there are generally 30 teeth in the oral cavity. There are 16 upper collar teeth, 14 lower collar teeth, and 26 breast teeth in the kitten's mouth. The edges of the cat's tooth crown are very sharp, and the surface of the cat's tongue has a drilling film. The surface of the drilling film forms many rough nipples, resembling files. Cats can use it to lick the meat attached to the bones. The cat's esophagus is a relatively straight tube located on the dorsal side of the trachea, consisting of the muscular layer, subdermal layer, and anvil membrane. The cat's stomach is a single stomach. The small intestine includes the duodenum, jejunum, and ileum. The short, wide, and thick characteristics of the catgut have distinct carnivorous characteristics.

Cats have well-developed sleeping fluid glands, which secrete a large amount of thin sleeping fluid when eating. Not only can it moisten food, facilitate swallowing and digestion, but the lysozyme in the sleeping fluid can also sterilize, disinfect, and deodorize the mouth, maintain oral hygiene, and prevent highly perishable and spoiled meat from harming the oral organs.

In general, defecation tracing of cats is done at a fixed time and location, and the frequency, shape, quantity, odor, and color of defecation are all stable. Although cats have been domesticated for a long time, their digestive anatomy and physiological structure still maintain the characteristics of carnivores. Therefore, a higher proportion of animal feed is added to the cat's feed to maintain normal digestive and physiological functions and ensure the need for nutrients.

2.2 Circulatory system

The circulatory system of cats is quite well developed, much like that of humans, and consists of a four-chambered heart, arteries, veins and lymphatic vessels. With 3 cusps in it, a cat’s heart is located on the left side of the anterior thorax, which is responsible for circulating blood to all parts of the body, in addition to regulating blood temperature. The blood of the cat is composed of liquid plasma and blood cells suspended in plasma, of which about 2/3 is plasma and 1/3 is blood cells. Blood is circulated in closed ducts by the heart's rhythmic contractions, thus delivering nutrients and carrying away waste to all parts of the body.

2.3 Respiratory system

The respiratory system of cats consists of the nasal cavity, the trachea, the lungs, and two small lung lobes. The respiratory system begins at the nostrils, involves several structures in the head, continues down to the neck, and finally reaches the lungs in the thoracic cavity (Figure 5). Its respiration is faster than that of humans, and it can breathe 30 to 40 times per minute.

In addition to breathing, the cat's respiratory system has other important functions, such as humidifying and heating the air before it enters the body, trapping and expelling foreign substances, promoting the sense of smell, and making sounds (e.g., meowing, purring).

2.4 Reproductive system

The reproductive system of cats consists of two gonads (female and male glands), four spermatic cords, a vagina and a phallus, which are capable of passing reproductive cells.

The male cat's reproductive system includes the testes, epididymis, vas deferens, prostate, urethra, scrotum and penis, but no seminal vesicles. Sperm are formed in the testes and go into the epididymis for storage. When mating and ejaculating, semen is sent from the penis to the vagina of the female cat via the vas deferens and urethra.

The reproductive system of the female cat includes the ovaries, fallopian tubes, uterus and vagina. The uterus is bicornuate and "v" shaped. The egg matures in the ovary, enters the fallopian tube and combines with the male cat's sperm to complete the fertilization process, then moves to the uterus to make the female cat pregnant.

2.5 Central nervous system

The central nervous system is the most important physiological system of the cat, centered on the brain and consisting of the cerebrum, brainstem and spinal cord, which is responsible for the coordination and control of all movements and behaviors in the body (Figure 6).

The cat brain is small, but it is as complex and capable as the human brain. The brain controls various behaviors through learning, motivation and perception. It generates neural impulses to move muscles, sends signals throughout the body, and controls many automatic bodily functions. The brain also receives and records sensory impulses. It is composed of billions of neurons, each with up to 10 000 connections to other cells.

The brainstem is located at the base of the brain and connects the cerebellum to the spinal cord. Almost all of the brain nerves that control various functions of the head come from the brainstem. It controls many basic life functions. Other parts of the brain include the pituitary gland, which coordinates and controls other glands, the olfactory bulb, which processes odors, the hypothalamus, which secretes hormones and controls the autonomic nervous system, and the pineal gland, which produces melatonin.

The spinal cord is the lower part of the central nervous system and is responsible for transmitting information between the brain and the body. It is a slender structure made up of major nerve bundles that carry nerve impulses. The spinal cord is connected to various parts of the body by nerves that leave through the gaps between the vertebrae and enter the spinal column. Its role is to coordinate motor and muscle activity, as well as to control automatic and voluntary reflexes.

2.6 Bones

Cats have a total of 230 to 247 bones and 500 muscles, more than those of humans and some animals (Fu, 1994). The most distinctive feature of the feet is the presence of five knuckles, which enable better grip on objects (Figure 7).

The bones of the cat's entire body include: skull, trunk bones, forelimb bones, hindlimb bones, and visceral bones. The skull consists of the harnpan and facial bones. The harnpan consists of paired frontal, parietal, and temporal bones and unpaired occipital, interparietal, pterygoid, and sieve bones. The facial bones consist of paired maxillae, incisors, nasal bones, zygomatic bones, tear bones, jaw bones, pterygoid bones, upper and lower turbinate bones, and unpaired plow bones, mandibles, and hyoid bones (Piras et al., 2013). The trunk bones consist of vertebrae. The forelimb bones consist of the scapula, humerus, forearm bones (ulna and radius), wrist bones, metacarpals, phalanges, and seed bones. The hind limb bones are divided into hip bone (sit bone, pubic bone, iliac bone), femur, patella, calf bone (tibia and fibula), tarsus, metatarsus, toe bone, and seed bone. The visceral bones are mainly the penis bones of male dogs.

2.7 Cat claws

Cats' claws are their most important weapon. They have four paws, of which the hind limb paws cannot be contracted, but the front limb paws can be contracted, and they also have a sharp "claw" that can help them catch prey.

Cats have fatty fleshy pads at the base of their paws, which have a cushioning and shock-absorbing effect, allowing them to walk without sound and jump without injury. The tips of the claws have very sharp nails that are curved and can be retracted. Cats put their claws away when walking and resting, and only extend them when hunting and climbing.

3 Physiological Sensory Characteristics of cats

3.1 Hearing

Cats have a very sensitive sense of hearing, which is mainly determined by the specific anatomy and physiology of its auditory organs. The ear is the auditory organ and consists of the outer, middle, and inner ear. Cats can hear sounds from 30 Hz to 45 kHz, while humans can only hear sounds from 20 Hz to 20 kHz. The cat's localization function for sound is also stronger than that of humans. Its two ears, like radar antennas, can rotate with the direction of sound and can distinguish between two similar sounds that are about 1 meter apart at a distance of 15 to 29 meters. In addition, the balance function of the cat's inner ear is much stronger than the balance function of the human inner ear.

3.2 Vision

Cats are binocular vision animals. A single cat eye has the field of view of 100 degrees, and both eyes can reach a wide field of view of 200 degrees. A cat's visual sensitivity is six times that of humans, and not only does it have good daytime vision, but its night vision is also extremely sharp, and it can clearly distinguish things even at night in a very low light environment.

The cat's eye is similar to the human eye, consisting of the cornea, iris, lens, retina, and other structures. Cats are unable to distinguish between rich colors as humans do, and their retinal cells are only green and blue. Thus, cats can only distinguish a limited number of colors, such as gray, green, blue, and yellow.

The cat's eyes look shiny in the dark, caused by the reflection of external light. In the cat's eye there are structures called photoreceptors, which are located in the middle layer of the choroid of the eye and consist of many closely packed cells, and this layer is able to reflect a portion of the incoming light back to the retina. Thus, the retina is equivalent to receiving dual irradiation, which greatly improves the animal's night vision.

3.3 Sense of smell

A cat's ability to smell is far beyond our imagination. Cats have more than 240 million olfactory cells, nearly 20 million more than dogs. According to statistics, cats have about 40~80 million microscopic olfactory receptors to recognize and process odors. This is why when the owner comes home from work, he hears the cat calling to the owner before he even reaches the door.

3.4 Sense of taste

Cats lost the sense of sweet taste due to genetic mutations in early evolution (Li et al., 2005), so they can only distinguish between sour, salty and bitter tastes. However, food is attracted to cats by smell, not taste. Cats rely on the smell of food to determine whether to eat it or not, and walk away if they do not like the taste; they do not put it in their mouths and taste it, as people do.

3.5 Sense of touch

The most developed sense of touch in cats is the whiskers, and the most sensitive part of the whiskers is the root of the whiskers, because the nerve cells are concentrated in this place. They have up to 30 whiskers on their faces, and when they cannot see objects, the whiskers will give them useful information. They also have whiskers on their feet, which are used to feel obstacles, terrain and the vital points of prey.

4 Conclusion and Progress

Cats have very well-developed physical body characteristics, like humans, with a self-contained digestive system, circulatory system, respiratory system, reproductive system, central nervous system, skeleton, and cat claws for predation feeding, digestion and absorption, respiratory excretion, transmission, and coordination of movement.

Cats have sensitive senses, much more sensitive than humans, and can perceive the external world more often, faster and more accurately, so we feel that it is a very alert animal. They can acquire information through sight, hearing, taste and touch. They are able to keenly identify subtle sounds, smells and changes when exploring their environment. Cats have an early heartbeat, a well-tolerated organism, high resistance to disease, and high tolerance to cold temperatures.

For human beings, cats make excellent pets. Cats are energetic enough and easy to be trained to help people reduce stress, improve their mood, enhance social skills and improve their relationship with each other. As human society continues to develop, the potential of cats as human pets will continue to be explored and tapped to meet the desires of different age groups and social groups to become one of man's closest friends.

Authors’ contributions

XJ was responsible for thesis conception, literature collection, thesis writing, revision and finalization; XMY was responsible for the translation of the paper, and HYP was responsible for the proofreading of the paper. All authors read and approved the final manuscript.

Acknowledgments

This study was supported by the Cuixi Innovation Research and Development Project Fund of Zhuji Cuixi Academy of Biotechnology.

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