116 



INTRODUCTION TO NEUROLOGY 



the word "dorsal." The head end of the body will be referred to as the 

 "anterior" or "cephalic" end; the other end of the body as the "posterior" 

 or "caudal" end. The terms "upper" or "higher" and "lower" will refer 

 to the relations in the erect human body. In the nomenclature of the 

 medulla oblongata (see p. 122) and of the thalamus (p. 167) our usage 

 departs slightly from that of the B. N. A. Regarding the naming of fiber 

 tracts see page 128. 



Figure 46 illustrates the form of the brain in a very early 

 human embryo. Its tubular form is very evident, and in the 



-Optic ocsicle 



Futvrt poritint 

 f/enure 



Pallium 

 Anterior neuropore 



Maencephalon 

 Isthmus 



Mtsencephahn 



Optic rtcai 



Future panting 

 Rhombtnciptidon flexure 



Fig. 46. An enlarged model of the brain of a human embryo 3.2 mm. 

 long (about two weeks old). The outer surface is shown at the left, and 

 on the right the inner surface after division of the model in the median 

 plane. The Anterior neuropore marks a point where the neural tube 

 is still open to the surface of the body. The Pallium is the region from 

 which the cerebral cortex will develop. The Optic recess marks the 

 portion of the lateral wall of the Diencephalon from which the hollow 

 Optic vesicle has evaginated. (After His, from Prentiss' Embryology.) 



brain the diameter of the tube is but little greater than that 

 of the spinal cord. The walls are thin and the cavity wide. 

 In a slightly older embryo the form is shown in Fig. 47, and 

 Fig. 48 illustrates diagrammatically the median section of an 

 embryo of about the same age as that shown in Fig. 47, upon 

 which the regions as denned by the B. N. A. are indicated. The 



