692 THE X KB VOLS SYSTEM 



superior scuneircular canal. The third impression (incisura temporalis) is situated near the 

 vallecula Sylvii, about one-third of an inch external to the tentorial groove. 



The LIMBIC LOBE lies on the mesial and tentorial surfaces of the cerebrum. It 

 ii? bounded externally by the calloso-marginal sulcus, the subparietal or post-limbic 

 sulcus, the stem of the calcarine tissure, and the collateral fissure. It consists of 

 an outer part formed by the callosal convolution, the isthmus and the uncinate 

 gyrus, and an inner part in which are included the fornix, the sei)tum lucidum, 

 the stria? longitudinales mediales and laterales, and the fascia dentata, all of which 

 will be subsequently described. 



THE OLFACTORY LOBE. — The olfactory lobe attains a considerable size in some 

 of the lower mammals, and in them it may contain a prolongation of the lateral 

 ventricle in its interior; in seals it is small, in cetacea it is absent, and in monkeys 

 and men it is rudimentary. Its anterior part is developed as an outgrowth of the 

 cerebral vesicle, but the cavity, which is present at first, soon disappears in man, 

 thougli its position is indicated, even in the adult, by the remains of the ependyma 

 which lined it. 



The olfactory lobe is divided into two lobules — the anterior and the posterior. 

 The constituent parts of the anterior olfactory lobule are the bulb, the tract, 

 the trigonum olfactorium, and the olfactory area of Broca. It is separated from 

 the posterior olfactory area by a curved fissure, fissura prima, which runs outwards 

 from the great longitudinal fissure in front of the anterior perforated space. 



The olfactory bulb is an ovoid mass of gray and white matter a little 

 more than a thircl of an inch long (1 cm.) and about a sixth of an inch (4 mm.) 

 wide. It rests below on the cribriform plate of the ethmoid bone, and is lodged 

 above in the olfactory sulcus on the orbital surface of the frontal lobe. About 

 twenty olfactory nerves spring from its lower surface and pass through the foramina 

 in the cribriform plate. 



The olfactory tract is a triangular band of white matter Avhich extends 

 backwards f]-om the posterior extremity of the bulb. It is about three-quarters of 

 an inch long (2 cm.) and one-tenth of an inch broad (2.5 mm.). Its apex is 

 embedded in the olfactory sulcus and its lower surface rests on the presphenoid 

 bone (juguni sphenoidale). Posteriorly it terminates in two roots, the mesial and 

 lateral, which enclose the trigonum olfactorium. The lateral root passes outwards 

 and backwards across the anterior perforated space to the anterior end of the 

 uncinate gyrus; the mesial root curves inwards behind Broca's olfactory area and 

 becomes continuous with the lower end of the callosal gyrus. The anterior olfac- 

 tor}' lobule is, therefore, intimately connected with the anterior part of the limbic 

 lobe, the two together forming the rhinencephalon, or racquet-shaped lobe of Broca. 



The trigonum olfactorium is a small triangular area of gray matter enclosed 

 between the two roots of the olfactory tract and bounded behind by the fissura 

 prima. 



Broca's olfactory area is a small part of the posterior end of the gyrus 

 rectus separated off from the remainder by a small oblique sulcus called the fissura 

 serotina and bounded ])osteriorly and externally l)y the mesial root of the olfactory 

 tract. 



The posterior olfactory lobule is that jxtrtion of the gray cortex which 

 forms the anterior perforated space. It is bounded internally by the peduncle of 

 the corpus callosum, anteriorly by the fissura prima which separates it from the 

 anterior olfactory lobule, and it extends externally into the vallecula Sylvii, being 

 crossed by the lateral root of the olfactory tract. 



The Corpus Callosum is a great white transverse commissure which unites the 

 cerebral hemisi)heres. Its form can be well studied in a mesial section of the brain 

 (fig. 412), in which it a])pears as a longitudinal arch with rounded anterior and 

 l)osterior extremities. The posterior extremity is called the splenium ; it is 

 rounded off ])ehind and is formed by the corpus callosum doul)ling on itself, so that 

 a small portion is folded forwards under, and in close opposition to, the larger part 

 (fig. 413). The anterior extremity or genu is less sharply bent than the poste- 

 rior end. It is continued downwards and backwards into a portion which appears 



