4 TEXT-BOOK OF ANATOMY. 



The numerous organs which form the various systems are themselves built up 

 of tissues, the ultimate elements of which can be studied only by the aid of the 

 microscope. The knowledge of these elements and of the manner in which they are 

 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 observa- 

 tion, and in order that precision and accuracy may be attained it is necessary that 

 we should be provided with a series of well-defined descriptive terms. It must 



FIG. 1. HORIZONTAL SECTION THROUGH THE TRUNK AT THE LEVEL OF THE FIRST LUMBAR VERTEBRA. 



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 

 the palms look forwards and the thumbs laterally. An imaginary plane of 

 section, passing longitudinally through the body so as to divide it accurately into a 

 right and a left half, is called the median 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 a left kidney and a right 

 and a 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 median 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 



