10 NERVOUS SYSTEM OF VERTEBRATES. 



Although the present book, as an introduction to the study of the 

 nervous system, will deal chiefly with structure it is hoped that 

 the student will be led into full sympathy with the functional point 

 of view in dealing with anatomical facts. 



One of the most serious questions before the student of the 

 nervous system at present is, what names and terms will serve best 

 for clear description. Although there is an unfortunate variance 

 in the usage of different writers certain terms are in more or less 

 common use and it is assumed that the reader is familiar enough 

 with the anatomy of vertebrate animals to understand the meaning 

 of these terms. For example, to indicate the two ends of a verte- 

 brate animal the terms front and hind, anterior and posterior, 

 cephalic or rostral and caudal ends are used, and for the relation 

 of direction toward the head or toward the tail' the convenient 

 terms cephalad and caudad are frequently used. So the terms 

 right and left, dextral and sinistral sides and dextrad and sinistrad 

 express corresponding relations, while the term median means in 

 the middle vertical plane which divides the body into right and 

 left halves. The terms mesial and lateral, mesad and laterad 

 are similar in meaning to those already mentioned. For the 

 upper and lower surfaces of vertebrate animals the terms dorsal 

 and ventral are regularly used and these together with the other 

 terms mentioned will be applied in the following pages to man in 

 the same sense as to other vertebrates. 



Since most of the description of the nervous system is taken 

 from microscopic sections it is necessary to understand the terms 

 used to designate sections cut in different planes. Sections made 

 at right angles to the longitudinal axis of the body are called trans- 

 verse sections. In many cases, as in embryos with the head bent 

 ventrad, sections transverse to the trunk will not be transverse 

 in the head region. Unless otherwise stated transverse sections 

 are understood to be at right angles to the longitudinal axis in the 

 region under consideration. Sections taken at right angles to 

 transverse sections and passing symmetrically through correspond- 

 ing organs of the right and left side, such as the limbs, would 

 be parallel with the horizontal plane in animals whose position 

 is prone. In man such sections would be approximately parallel 



