10 ELEMENTARY PHYSIOLOGY. less. 



or cavity of the skull, opens into the spinal canal. It 

 contains a mass of nervous matter called the brain, which 

 is continuous with the spinal cord, the brain and the 

 spinal cord together constituting what is termed the 

 cerebro-spinal system (Fig. 1, G.S., C.S.). The 

 ventral chamber, or cavity of the face, is almost entirely 

 occupied by the mouth and pharynx, into which last 

 the upper end of the alimentary canal (called gullet or 

 CBSOphagUS) opens. 



Thus, the study of a longitudinal section shows us 

 that the human body is a double tube, the two tubes being 

 completely separated by tlie spinal column and the bony 

 axis of the skull, which form the floor of the one tube and 

 the roof of the other. The dorsal tube contains the cere- 

 bro-spinal axis ; the ventral tube contains the alimentary 

 canal, the sympathetic nervous system, the heart, and the 

 lungs, Ijesides other organs. 



Transverse sections, taken perpendicularly to the axis 

 of the vertebral coluu\n, or to that of the skull, show still 

 more clearly that this is the fundamental structure of the 

 human body, and that the great apparent difference be- 

 tween the head and the trunk is due to tlie different size 

 of the dorsal cavity relatively to the ventral. In the head 

 the former cavity is very large in proportion to tlie size of 

 the latter (Fig. 1, B) ; in the thorax, or abdomen it is 

 very small (Fig. 1, C). 



The limbs contain no such chambers as are found in 

 the body and the head ; but with the exception of certain 

 brandling tubes filled witli fluid, which are called blood- 

 vessels and lymphatics, are solid or semi-solid, 

 throughout. 



3. The Tissues generally. — Such being the general 

 character and arrangement of tlie jiarts of the human 

 body, it will next be well to consider into what constitu- 

 ents it may be separated by the aid of no better means of 

 discrimination than the eye and the anatomist's knife. 



With no more elaborate aids than these, it becomes 



