‘Anatomy’ and ‘physiology’ are scientific terms used to describe the study of the structure of the body (anatomy) and how the body actually ‘works’ (physiology). In this section, we will study the anatomy and physiology of the dog and cat. In Section 2, we describe the comparative anatomy and physiology of the horse, the most common farm animals and some of the most commonly kept exotic species. We start by looking at the basic unit of the body – the cell – and then work our way through the tissues, organs and systems until the picture is complete.
Animal classification
When studying any aspect of biology it is important to have a basic understanding of the classification system used to group animals. How the species that one may meet in a veterinary practice fit into this classification system should also be understood. Classification is the way in which we ‘sort’ species into orderly groups, depending on how closely they are related in terms of their evolution, structure and behaviour. The science of classification is known as taxonomy.
If organisms have certain basic features in common they are grouped together into a kingdom. For example, if an organism is composed of more than one cell (i.e., it is multicellular) and obtains its food by ingestion, it is placed in the animal kingdom. Other kingdoms include plants and fungi. The animal kingdom is then further subdivided, based upon similarities of organisms, into a hierarchical system (Table 1.1). This narrows the classification down until we eventually reach a particular genus and species. Most living organisms are identified by a genus and species – a method known as the binomial system, invented by the Swedish scientist Carl Linnaeus.
Table 1.1
Classification of the domestic dog and cat
| Taxonomic group | Dog | Cat |
| Kingdom | Animal | Animal |
| Phylum | Chordata (vertebrate) | Chordata (vertebrate) |
| Class | Mammalia (mammal) | Mammalia (mammal) |
| Order | Carnivora | Carnivora |
| Family | Canidae | Felidae |
| Genus | Canis | Felis |
| Species | familiaris | catus |
| Common name | Domestic dog | Domestic cat |
All the species within the animal kingdom are divided into those with backbones – the vertebrates – and those that do not have backbones – the invertebrates (e.g., insects, worms, etc.). The vertebrates are divided into eight classes. The classes that are of the most veterinary importance are:
• Amphibia – approximately 3080 species
• Reptilia – approximately 6600 species
• Aves or birds – approximately 8500 species
• Fish – approximately 30,000 species.
• Mammalia – approximately 4070 species.
These classes are then further divided into orders, and so on, until a species is identified, as in Table 1.1.
Most of this section of the book concerns the mammals, because the majority of animals seen in veterinary practice will be from this class. The distinctive features of mammals are the production of milk by the mammary glands and the possession of hair as a body covering. Examples of mammalian orders include:
• Insectivores (e.g., shrews, moles)
• Rodents (e.g., mice, rats)
• Lagomorphs (e.g., rabbits, hares)
• Carnivores (e.g., cats, dogs, bears, seals)
• Ungulates (e.g., cows, sheep, horses)
• Cetaceans (e.g., whales, dolphins)
• Primates (e.g., monkeys, apes)
Generally speaking, all mammals have a similar basic structural plan in terms of anatomy and physiology, but each species has been modified to suit its specific lifestyle. In other words, mammals have become specialised for activities such as running, digging, gnawing, jumping and eating specific foods.
Anatomical definitions
When studying anatomy and physiology it is important to understand the terms that are ...
