4 TEXT-BOOK OF ANATOMY. 
grouped together to form the various tissues of the body forms an important branch 
of anatomy, which is termed histology. 
The structure of the human body may be studied in two different ways. The 
several parts may be considered with reference to their relative positions, either as 
they are met with in the course of an ordinary dissection, or as they are seen on 
the surface of a section through the body. This is the topographical method. On 
the other hand, the several systems of organs may be treated separately and in 
sequence. This constitutes the systematic method, and it is the plan which is 
adhered to in this treatise. 
Descriptive Terms.—Anatomy is a descriptive science founded on observation, 
and in order that precision and accuracy may be attained it is absolutely necessary 
that we should be provided with a series of well-defined descriptive terms. It must 
be clearly understood that all descriptions are framed on the supposition that the 
body is in the erect. position, with the arms by the side and the hands held so that 
Fia. 1.—HorRIZONTAL SECTION THROUGH THE TRUNK AT THE LEVEL OF THE First LUMBAR VERTEBRA, 
the palms look forwards and the thumbs outwards. An imaginary plane of 
section, passing longitudinally through the body so as to divide it accurately into a 
right and left half, is called the mesial plane, Fig. 1 (M.P.). When the right and 
left halves of the body are studied it will be found that both are to a large extent 
formed of similar parts. The right and left limbs are alike; the right and left 
halves of the brain are the same; there are a right and left kidney and a right and 
left lung, and so on. So far, the organs are said to be symmetrically arranged. 
But still a large amount of asymmetry may be observed. Thus the chief bulk of 
the liver lies to the right side of the mesial plane, and the spleen is an organ 
which belongs wholly to the left half of the body. Indeed it is well to state that 
perfect symmetry never does exist. There always will be, and always must be, a 
certain want of balance between symmetrically placed parts of the body. Thus 
the right upper limb is, as a rule, constructed upon a heavier and more massive 
plan than the left, and even in those organs where the symmetry appears most 
perfect, as for instance the brain and spinal cord, it only requires a closer study to 
reveal many points of difference between the right and left halves. The line 
on the front of the body along which the mesial plane reaches the surface is 
