=The Nervous System.=--In so far as our present purposes are concerned, the nervous system may be disposed of in a few words. The central nervous system consists of a brain and spinal cord. The microscopic elements of this tissue are peculiarly modified cells, consisting of a central body, from which fibers run in two or more directions. The cell bodies constitute the gray matter, and the fibers the white matter of the brain and spinal cord. The gray substance is inside the spinal cord and on the surface of the brain, constituting the cortex. The most important parts of the brain are the cerebrum, optic lobes, cerebellum, and medulla. There are twelve pairs of cranial nerves originating in the brain and controlling the special senses, movements of the face, respiration, and pulse rate. From each segment of the spinal cord a pair of spinal nerves arises, each of which possess both sensory and motor roots. The sympathetic nervous system consists of a trunk on either side, running from the base of the skull to the pelvis, furnished with ganglionic enlargements and connected with the spinal nerves by small fibers. =The Respiratory Organs.=--These include the nose, larynx, trachea or windpipe, and lungs. The trachea forks into bronchi and bronchioles of smaller and smaller size, ending in the alveoli or blind sacs of the lungs. In fowls there are numerous extensions of the respiratory system known as air sacs, and located in the body cavity and also in the hollow bones. The air sacs communicate with the lungs, but not with one another. =The Urinary Organs.=--These consist of kidneys connecting by means of ureters with a bladder from which the urethra conducts the urine to the outside. In the male the urethra passes through the penis and in the female it ends just above the opening of the vagina. The kidneys are usually inclosed in a capsule of fat. The right kidney of the horse is heart-shaped, the left bean-shaped. Each kidney of the ox shows 15 to 20 lobes, and is oval in form. The kidneys of sheep, goats, and swine are bean-shaped and without lobes. =The Reproductive Apparatus.=--This consists of ovaries, oviducts, uterus or womb, and vagina in the female; the testes, spermatic cords, seminal vesicle and penis, together with various connecting glands, especially prostate gland and Cowper’s gland, in the male. In fowls there is no urinary bladder, but the ureters open into the cloaca or posterior part of the rectum. The vagina and uterus are also wanting in fowls, the oviducts opening directly into the rectum. The male copulating organ is absent except in ducks, geese, swan, and the ostrich. CHAPTER II Some Physiology You Ought to Know