INTRODUCTION

Mammals obviously need no introduction to any veterinary surgeon! This section is intended to revise the main mor­phological and physiological differences between reptiles, birds and mammals.

Mammals are endothermic, with tactile vibrissae and a pinna for collecting sound. The skin has hair modified into fur, spines, wool, and even scales. Sebaceous glands prevent water loss while sweat glands play a role in insulation through evaporation of water and salts. They have separate reproductive and excretory passages and, except for the more primitive monotremes, no cloaca. They have loops of Henle, which enable them to produce concentrated urine, and the end point of nitrogenous waste is urea. Marine mammals do not have salt glands but osmoregulate and excrete salt via their kidneys (King & Custance 1982; Vaughan 1986b).

It is thought that the high mammalian body temperature is too high for sperm development so the testes migrate outside the body to a cooler location in the scrotum. In placental mammals the lower oviduct is modified into a uterus where the embryo is nourished. Three types of uterus are found (Fig. 7.1). The primitive duplex uterus found in the rabbit, rat, and hamster has two separate branches and separate cervices uniting at the vagina. In most mammals like carnivores (dog and cat), guinea pigs, and ungulates the distal part of the uterus fuses into a uterine body to form the bicornuate or bipartite (slight partition remaining) state. The most advanced form is found in primates where there is total fusion of the uterine body, forming what is paradoxically called the simplex state (King & Custance 1982).

The heart is completely divided into four chambers, which enables the maintenance of a high blood pressure. The aortic arch curves to the left in mammals. They have a diaphragm that separates the abdomen from the thorax and extensive nasal turbinates to aid in thermoregulation.

Mammals have a specialized atlas-axis complex to help them rotate their head up and down and side to side. Ribs are restricted only to the thoracic vertebrae, thus enabling them to twist their lumbar spine in dorsoventral flexion. The skull has a large double occipital condyle and the adductor jaw muscles, which open the mouth, are divided into the masseter and the temporalis (King & Custance 1982). The temporal fossa provides attachment for the temporalis while the masseter inserts on the zygomatic arch. Carnivores have well developed temporal muscles while in herbivores the masseter muscle is the main muscle of mastication and the jaw articulation lies dorsal to the occlusal surfaces of the teeth (Crossley 2003) (Fig. 7.2). This enables herbivorous mammals like rabbits to have the side to side, forward, backward, and rotary chewing actions that grind down grasses. The mandible is formed from a single dentary bone to allow for insertion of these powerful muscles; the quadrate and articular bones have become the ear ossicles, incus, and malleus respectively (King & Custance 1982).