[Illustration:  Fig. 8.—­A longitudinal section through the middle of the body showing the external and internal surfaces and the organs.

1.  The skull. 2.  The brain, showing the convolutions of the gray exterior in which
    the nerve cells are most numerous.
3.  The white matter in the interior of the brain formed of nerve
    fibres which connect the various parts of this.
4.  The small brain or cerebellum. 5.  The interior of the nose.  Notice the nearness of the upper part of
    this cavity to the brain.
6.  The hard or bony palate forming the roof of the mouth. 7.  The soft palate which hangs as a curtain between the mouth and the
    pharynx.
8.  The mouth cavity. 9.  The tongue. 10.  The beginning of the gullet or oesophagus. 11.  The larynx. 12.  The windpipe or trachea. 13.  The oesophagus. 14.  The thyroid gland. 15.  The thymus gland or sweetbread. 16.  The large vein, vena cava, which conveys the blood from the brain
    and upper body into the heart.
17-25.  Lymph nodes; 17, of the neck; 25, of the abdomen. 18.  Cross section of the arch of the aorta or main artery of the body
    after it leaves the heart.
19.  The sternum or breast bone. 20.  The cavity of the heart. 21.  The liver. 22.  The descending aorta at the back of the abdominal cavity. 23.  The pancreas. 24.  The stomach. 26.  Cross section of the intestines. 27.  The urinary bladder. 28.  The entrance into this of the ureter or canal from the kidney. 29.  Cross sections of the pubic bone. 30.  The canal of the urethra leading into the bladder. 31.  The penis. 32.  The spinal cord. 33.  The bones composing the spinal column. 34.  The sacrum.  The space between this and No. 29 is the pelvis. 35.  The coccyx or extremity of the back bone. 36.  The rectum. 37.  The testicles.]

Between these various surfaces is the real interior of the body, in which there are many sorts of living tissues,[2] each, of which, in addition to maintaining itself, has some function necessary for the maintenance of the body as a whole.  Many of these tissues have for their main purpose the adjustment and cooerdination of the activities of the different organs to the needs of the organism as a whole.  The activity of certain of the organs is essential for the maintenance of life; without others life can exist for a time only; and others, such as the genital glands, while essential for the preservation of the life of the species, are not essential for the individual.  There is a large amount of reciprocity among the tissues; in the case of paired organs the loss of one can be made good by increased activity of the remaining, and certain of the organs are so nearly alike in function that a loss can be compensated for by an increase or modification of the function of a nearly related organ.  The various internal parts are connected by means of a close meshwork of interlacing fibrils, the connective tissue, support and strength being given by the various bones.  Everywhere enclosing all living cells and penetrating into the densest of the tissues there is fluid.  We may even consider the body between the surfaces as a bag filled with fluid into which the various cells and structures are packed.