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CHAPTER I. 



THE OSSEOUS SKELETON. 



The whole body of the sole is covered by the skin, which as everyone knows is so slightly 

 attached to the parts beneath it that it can be stripped off as a definite continuous 

 membrane. Beneath it is found the flesh, wliich consists of the muscles, by the 

 contraction of which all the movements of the fish are produced. The muscles are 

 attached to the bones which together form the skeleton. In the middle region of 

 the body on the ventral side is a cavity with smooth walls, within which are contained 

 the entrails, or viscera, consisting of the organs of digestion, excretion, and 

 reproduction. The principal organ of digestion is the digestive tube which leads from 

 the mouth and after various convolutions opens to the exterior again at the vent or 

 anus. The organs of respiration or gills are fringes supported on rods between which 

 are slits or clefts by which the cavity of the throat opens on each side into a gill 

 chamber. Each gill chamber again opens by a single aperture to the exterior. The 

 blood vessels ramify in the substance of aU the organs now mentioned, but the heart 

 which keeps the blood moving in circulation is contained in a special cavity which is 

 separate from that containing the viscera. The skin and the membrane lining the 

 digestive tube become continuous with one another at the mouth and anus, and at the 

 gill-clefts. The nerves also ramify in the substance of the muscles and other organs, 

 but, excepting the sympathetic system, they all radiate from the brain and spinal cord 

 which are contained in a chamber on the dorsal side of the skeleton, the brain being 

 contained in the cavity of the skull, the spinal cord being enclosed by a series of 

 arches formed by processes of the spine. Some of the nerves convey the impressions 

 received by the senses to the brain, and may be regarded as proceeding from the sense 

 organs to the brain ; the principal sense organs are the skin, the e3'es, the ears, and 

 the olfactory organs. The other nerves conduct impulses from the brain and spinal 

 cord to the muscles and other organs causing the former to contract and regulating 

 the functions of the latter. 



The structure and relations of these various organs can be made most easily 

 intelligible by describing the skeleton first. 



The Skull. — -The central part of the skeleton consists of tlie skull and the vertebral 

 column, the latter being composed of a longitudinal series of distinct bones, the 



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