THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 231 



tinct areas. The sulci develop in more or less definite relation to these areas, 

 the great majority making their appearance along the boundary lines between 

 them. These are known as terminal sulci, of which the rhinal fissure and central 

 sulcus are examples. Sometimes the folding occurs entirely within such an 

 area, i. e., along its axis, producing what is known as an axial sulcus. But 

 there are still others in which the relation to these functional areas is not so evi- 

 dent. The arrangement of the fissures and sulci in a seven month fetus is shown 

 in Fig. 164. 



The Development of the Septum and Commissures. The two hemispheres 

 are connected by the lamina terminalis, which serves as a bridge for fibers which 

 cross from one hemisphere to the other. These form three important bundles: 



Fig. 165. Schematic representation of the development of the septum pellucidum and 

 telencephalic commissures: A. C., Anterior commissure; C. C., corpus callosum; C. F., columna 

 fornicus; C. S. P., cavum septi pellucidi; F., fornix; H. C., hippocampal commissure; /. F., 

 interventricular foramen; Fis., chorioid fissure; L. T., lamina terminalis. (Based on drawings of 

 models of the telencephalon of a four months' fetus (.4) and of a five months' fetus (B) by Streeter.) 



the anterior commissure, the hippocampal commissure, and the corpus callosum. 

 The two former connect the olfactory portions of the hemispheres, while the 

 latter is the great commissure of the non-olfactory cortex or neopallium. Every- 

 one admits that the anterior commissure develops in the lamina terminalis 

 (Fig. 165); and the corpus callosum and hippocampal commissures are said to 

 form in its dorsal part (Streeter, 1912). According to this account the lamina 

 terminalis becomes stretched by the great development of the corpus callosum 

 and appropriates part of the paraterminal body. This is the portion of the 

 rhinencephalon that lies immediately rostral to the lamina terminalis in the 

 medial wall of each hemisphere. Eventually the lamina terminalis presents a 

 large cut surface in the median sagittal section and includes the commissures 



