Senses

The dog's brain synthesizes, interprets, and acts on all the information it receives from the senses. Although the brain of an average dog accounts for less than half a per cent of its body weight, it needs a great deal of nourishment to function properly and receives over 20 per cent of the blood pumped out by the heart. Brain activity is, in part, predetermined by the "fixed wiring" of the dog's genes. Just as our brains are pre-wired to learn language, the dog's brain is pre-wired to learn to interpret scent, and a large part of the brain is devoted to this process. It is also able to interpret information from the other senses - touch, taste, hearing, and sight.  

A Beagle's compact size coupled with it's extraordinary sense of smell makes it a superb choice for work in special services or as a detector dog for termites, bedbugs, mealy bugs, home mold, building mold, winery mold, human remains, gas pipeline leaks, water leaks, endangered species, explosives, guns, drugs, gold ore, sea turtle eggs, traces of flammable compounds used in arson, brown tree snakes hiding in cargo bound for Hawaii, gypsy moth larvae, avalanche rescue, estrus (heat cycle) in cows, underground water leaks, crisis-response, according to experts in the field, weeds that are hazardous to agriculture, and foods that may carry dangerous insects and diseases into the United States. Cancer sniffing dogs are now being used to assist in medical research.

 

COORDINATION OF THE SENSES

Hearing: Highly mobile ears "capture" sounds and funnel them down to the eardrum. A dog might cock one ear to capture an initial sound, then use both ears to catch the maximum number of sound waves. Experiments show that a dog can can locate the source of a sound in six-hundredths of a second.

Vision: This dog's facial anatomy is similar to that of the wolf, with widely spaced eyes for good lateral vision. Focusing on an image directly in front is most efficient in breeds with frontally placed eyes.

Taste and Scent: Scenting and associated taste are both chemical senses. The average dog has over 225 million scent receptors in its nasal folds compared to a human's 5 million. It also has a vomeronasal organ above the roof of the mouth for capturing sex scent and transferring it to the brain.

The brain: All sensory information is converted in sensory nerves to chemical messages for transmission and analysis by the brain. Some of these messages influence the pineal gland, in the base of the brain, which synchronizes all body rhythms.

Touch: Touch receptors exist all over the body, but especially on the feet. 

HOW THE BRAIN WORKS: The dog's brain stores information in two ways - it can either be conditioned or it can store what it learns. Both responses rely upon the individual dog's information-storage systems and are, in part, determined by genetics. The brain consists of billions of cells (neurons), each of which may have up to 10,000 connections with other cells. The cells chemically communicate with each other through neurotransmitters. The speed of these transmissions depends partly upon a fatty substance called myelin. In the dog's prime, messages are transmitted at great speed, but as the brain ages, messages move more slowly. Anatomically, the dog's brain is similar to the brains of most other mammals. The cerebrum controls learning, emotions, and behavior, the cerebellum controls the muscles, and the brain stem connects to the peripheral nervous system. Each sense feeds into the brain through its own special nerves. A network of cells throughout the brain (the limbic system) almost certainly integrates instinct and learning. The comfort between what a dog instinctively wants to do and what we teach it to do probably takes place in the brain's limbic system. Humans can override this system by giving rewards to the dog for obeying its owners rather than its "instincts". 

HOW THE SENSES WORK: The dog's senses are similar in function to those of humans. Information from the senses feeds into the brain, where it triggers either a body response or hormonal activity. For example, if a dog steps on something and feels pain, it quickly steps back - a physical response. If it smells male or female dog scent, the pituitary gland in its brain activates and stimulates a hormonal response.  

SIGHT: A dog's eye are flatter than a human's; although the dog can change the shape of its lenses, thereby adjusting focal length, it cannot do so as effectively as a human. A dog's eyes are more sensitive to light and movement than those of a human, but their resolving power is correspondingly less efficient. The consequence is that a human finds it easier to see a lost tennis ball lying in the grass than a dog does, whereas a dog finds it easier to see slight movement out of the corners of its eyes than a human does. 

 THE ANATOMY OF THE SENSES  

The eye

The eye consists of the corned, then an anterior fluid-filled chamber, followed by the three-part uvea - iris, ciliary body, and lens. Behind the lens is a large, fluid-filled posterior chamber, then the light sensitive retina which feeds information down the optic nerve to the brain. The third eyelid, which is hidden by the lower lid, sweeps the eye clean. The lachrymal gland produces tears to keep the cornea moist. Tears drain down the lachrymal duct into the nasal cavity. This can block, causing tears to overflow.

 

The ears


HEARING: The cartilaginous outer ear (pinna) captures sound, and funnels it down the external ear canal to the ear drum (tympanic membrane). Eardrum vibrations stimulate the organs of balance in the middle ear - the hammer, anvil, and stirrup (malleus, incus, and stapes), which amplify and transmit sound, while at the same time protecting the inner ear from excessive vibration. The cochlea (part of the inner ear) captures these sounds and converts them to chemical signals to the brain. Adjacent to the cochlea are the semicircular canals - the saccule and utricle - the organs of balance in the inner ear, which feed the brain with information on the alignment of the head.

 

The taste buds

TASTE: Most taste receptors are on the anterior portion of the tongue and are sensitive to sweet,  sour, bitter, and salty tastes. Other nerve endings act as touch or texture receptors. Although there are probably fewer than 2,000 taste receptors on the typical dog's tongue, as a "chemical" sense taste works in conjunction with the dog's infinitely more sensitive sense of smell. Odor initially attracts a dog to food, then taste and texture receptors take over.   

The scent receptors

SMELL: Moisture on the nose helps to capture scent, which is transmitted onto the nasal membranes, which cover the nose's wafer-thin turbinate bones. These bones have convoluted folds, ensuring that the tiniest amount of scent is captured within them. Sensory cells are closely packed along the nasal-membrane lining. Depending on the breed of the dog, smaller noses have less room for sensory cells. They convert scent to chemical messages transmitted to the olfactory bulb region of the brain. Other scents are captured by the vomeronasal organ above the roof of the mouth, and transmitted to other parts of the brain.

Though the size of this surface varies with the size and length of the dog's nose, even flat-nosed breeds can detect smells far better than people. The following table shows the number of scent receptors in people and several dog breeds. A dog's brain is also specialized for identifying scents. The percentage of the dog's brain that is devoted to analyzing smells through selective breeding, is actually 40 times larger than that of a human! It's been estimated that dogs can identify smells somewhere between 1,000 to 10,000 times better than nasally challenged humans can.

 

Scent-Detecting Cells in People and Dog Breeds

Species

Number of Scent Receptors

Humans
5 million
Dachshund
125 million
Fox Terrier
147 million
Beagle
225 million
German Shepherd
225 million
Bloodhound
300 million