228 COMPARATIVE ANATOMY 



involution of the median wall of the hemisphere, is much more 

 marked than in lower forms : the line of involution is known as 

 the hippocampal fissure. 



The central olfactory apparatus (rkincnccphalon) in its entirety 

 is represented by the olfactory bulb, peduncle, and tubercle, the 

 piriform lobe, and the hippocampus, and is separated from the 

 pallium by the rhinal fissure (Figs. 170, 171). This fissure is in 

 close relation to the splenial (cattosal) fissure, which bounds the 

 supracallosal gyrus dorsally, extending more or less parallel to the 

 corpus callosum. The Sylvian fossa or fissure is also a typical 

 fissure : it is situated at about the middle of the rhinal fissure, 

 and in the higher Mammals is overlapped by the pallium so that 

 the fossa is converted into a fissure. In Carnivores, Cetaceans, and 

 Ungulates, three gyri arch over the Sylvian fissure, one above the 

 other, and are separated by the so-called arcuate fissures (Fig. 171). 

 The upper of these, bounded above by the longitudinal pall in I 

 fissure, is spoken of as the marginal gyrus. Along the lateral sur- 

 face of the hemisphere, the cruciate sulcus (the homologue of the 

 central sulcus or fissure of Rolando of Primates) extends upwards 

 to the pallial fissure. Characteristic of the brains of all Apes except- 

 ing those with smooth hemispheres is the parieto-occipital sulcus, 

 between the parietal and occipital lobes : in Man, the lateral parts 

 of this fissure are more or less indistinct (cf. 173, B, in which other 

 gyri and sulci of the human brain are shown). 



The corpus striatum is surrounded and perforated by fibres 

 passing down from the pallium (anterior limb of the internal 

 capsule of Primates). Unlike the corresponding structure in 

 other Vertebrates, the corpus striatum of Mammals becomes 

 gradually more deeply situated, and is comparatively small as com- 

 pared with the rest of the brain. 



The olfactory lobes usually extend forwards freely from the base 

 of the telencephalon, and each may retain throughout life a 

 prolongation of the lateral ventricle (e.g. Perissodactyles) ; in 

 some cases (e.g. numerous aquatic forms and Primates) they are 

 completely covered by the frontal lobes. The degree of their 

 development is in proportion to that of the olfactory sense, and 

 they may even be entirely reduced (cf. under Olfactory Organ). 



The pineal body is displaced downwards by the hemispheres 

 and lies against the anterior lobes of the mid-brain, not reaching 

 to the roof of the skull and brain-membranes. Its bifurcated 

 peduncle connects it with the roof of the diencephalon and 

 contains nervous substance : its distal end has the form of a 

 rounded or oval sac, consisting of compact epithelial tissue and 

 containing concretions. A parietal organ is wanting. Traces of 

 the saccus vasculosus and lobi inferiores still occur, even in Man, 

 in connection with the infundibulum. 



The mid-brain (corpora bigemina) is of smaller relative size 

 than in other Vertebrates. A transverse furrow across the solid 



