STRUCTURE OF THE LOBES OF THE CEREBRUM. 399 



nerve cells. The fifth ventricle, or camera septi pellucidi, is 

 destitute of epithelium, like the median surface of the cerebral 

 lobes of which it is a part. 



5. THE FORMATION OF THE BULBUS OLFACTORIUS. The ante- 

 rior half-arch of the median constricting ring of the cerebrum 

 (see p. 379) is termed the olfactory lobe, or in Man, impro- 

 perly, the nervus olfactorius. It is a diverticulum of the sac 

 of the cortex, and is hollow, like the cerebral lobes, its cavity 

 communicating with the lateral ventricle (fig. 238, V). Above, 

 its cortex is directly continuous with that of the cerebrum gene- 

 rally, whilst below and behind (towards the lamina perforata 

 antica) it would immediately form a portion of the free border 

 of the cortex, if the lamina perforata antica did not expand 

 into an investment, to be subsequently mentioned, of cortical 

 substance, covering the basal surface of the nucleus caudatus. 



The bulbus olfactorius forms a cap superimposed upon this 

 conical process of the cerebrum (fig. 238, R'), from the external 

 surface of which, as is well known, are given off the nerves to 

 the Schneiderian membrane ; from its hollow internal surface 

 proceeds a layer of medullary fibres, which invests the anterior 

 surface of the olfactory lobe like a membrane, divides along the 

 external and internal convolutions of the olfactory lobe, and, as 

 seen from the basal aspect of the olfactory lobe in Man, has led 

 to its receiving the erroneous appellation of a nerve (m). 



The olfactory lobe divides on both sides of the anterior 

 perforated space into an internal and an external olfactory 

 convolution. The former is continuous with the frontal end 

 of the gyrus fornicatus, beneath which it may be recognized 

 for some distance as a separate longitudinal elevation (fig. 241, 

 RV). The latter, which is the largest, coalesces with the 

 temporal extremity of the above-described gyrus, the subi- 

 culum cornu Ammonis (fig. 242, Ra, Sub). 



The bulbus olfactorius, which Luys rightly declared to be 

 analogous to the retina, receives the brush of olfactory nerves 

 terminating peripherically in the olfactory cells as a short 

 projection system, just as the retina presents a still shorter 

 projection system, to wit, the connecting fibres between the 

 rods and cones as terminal organs, and their nervous elements 



