4 ELEMENTARY ANATOMY. [LESS. 



The second, and much smaller cylinder, runs along the 

 back, and consists of the backbone, expanding above into 

 the skull (or brain-case). This cylinder contains the spinal 

 marrow, while its upper expansion contains the brain. The 

 trunk of the human body consists thus of two tubes, with a 

 solid partition between them formed of the front part of the 

 backbone. Neither of these cavities is prolonged into the 

 limbs, which are made up of solid structures (flesh, nerves, 

 and vessels) wrapped round bones. 



6. The long alimentary tube has no communication with the 

 body-cavity which surrounds it, but is (with its glandular ad- 

 juncts) a continuous structure, except at its terminal openings. 



Thus it is not the inside of the alimentary tube which is 

 the true body-cavity, but, on the contrary, the space which 

 surrounds that tube and the other viscera. The tube itself is, 

 as it were, but a reflexion inwards of the external surface, the 

 skin which lines it being continuous at the lips with the skin 

 of the outside of the body. 



At its upper end this tube rather bends away from the 

 brain or upper termination of the central part of the nervous 

 system. 



The heart, which is a hollow muscular organ, is rhythmically 

 contractible and propulsive, and contains red blood ; part of 

 which, as it circulates, undergoes on its way back a subsidiary 

 (portal) circulation through the liver, by means of a double 

 set of vessels ramifying through that organ. 



The blood-vessels which arise from the heart (i.e. arteries) 

 become successively smaller and smaller as they pass away 

 from it, and end in most minute tubes (capillaries), whence the 

 returning vessels (veins) take origin, growing larger as they 

 approach the heart. The blood is thus constantly enclosed 

 in distinct vessels of one kind or another. 



The brain and spinal marrow form the central parts of the 

 nervous system. Cords or threads of nervous substance (the 

 nerves) extend from the brain and spinal marrow into every 

 part of the body. 



The lungs (as has been shown in "Elementary Physiology," 

 Lesson IV.) respire air, but man has not any appliance by 

 which to extract oxygen from air as it exists mixed up in 

 water, whether fresh or salt. 



The sense organs, except touch, are all placed in the head, 

 and the respiratory organs open at the same part of the body, 

 namely, by the nose and mouth. Three special senses are fur- 

 nished with pairs of organs two eyes, two ears, two nostrilc. 



