. Animal biology. Zoology; Biology. 408 METAZOAN PHYLA The alimentary canal shows several distinguishing characteristics. The mouth cavity is separated from the nasal chambers by a hard palate, which is a shelf of bone covered by soft tissues (Fig. 219). This is supplemented posteriorly by a fleshy soft palate. The passage from the mouth into the pharynx is known as the fauces, on each side of which lie the tonsils. The latter are masses of lymphoid tissue and their significance is not definitely known. The opening from the pharynx into the windpipe is the glottis. It is protected from the ent
. Animal biology. Zoology; Biology. 408 METAZOAN PHYLA The alimentary canal shows several distinguishing characteristics. The mouth cavity is separated from the nasal chambers by a hard palate, which is a shelf of bone covered by soft tissues (Fig. 219). This is supplemented posteriorly by a fleshy soft palate. The passage from the mouth into the pharynx is known as the fauces, on each side of which lie the tonsils. The latter are masses of lymphoid tissue and their significance is not definitely known. The opening from the pharynx into the windpipe is the glottis. It is protected from the entrance of food during swallowing by a fleshy and cartilaginous cover called the epiglottis. At the junction of the small and large intestines the latter is prolonged into a blind sac called the caecum, which is enlarged in herbivorous mammals, where its purpose seems to be to increase the capacity of the intestine. -Po/p cavii-y The contracted tip of this caecum is known ^Enamel. penfine Cemen1-um Nerve Bloodvessel in certain mammals, including man, as the vermiform, appendix. Excepting in the egg- laying monotremes, there is no cloaca, the anal opening being at the surface of the body. The lungs are contained in coelomic spaces called pleural cavities, which are the lateral portions of the thoracic cavity, the middle part of which is the pericardial cavity. The thoracic cavity is separated from the abdominal cavity by a thin, mus- FiG. 278. Canine tooth of a mam- c^i^^r diaphragm, which is convex anteri- mal, diagrammatic. " orly and concave posteriorly. The thoracic cavity is expanded in breathing by raising the ribs and flattening the diaphragm. Thus air is drawn into the lungs. By the relaxation of the muscles of the ribs and diaphragm and the resulting contraction of the thoracic cavity, air is forced out. At the upper end of the windpipe, or trachea, lies a larynx, or voice box. Mammals are warm-blooded animals possessing a well-developed heat-regulatory mechanism. Th
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