THE OLFACTORY LOBE. 159 



form a rudimentary gyrus (bordering gyrus, gyrvs marglnalis* Germ. Randwindung), which is 

 curved around the brain-stem and the central parts of the hemisphere, lying within and concen- 

 tric with the larger and well-developed gyrus formed by the fornicate and hippocampal gyri. 

 The lamina of the septum lucidum, and the so-called peduncle of the corpus callosum (gyrus 

 subcallosus of Zuckerkandl) also belong to this bordering gyrus, but have become separated 

 from the supracallosal part by the development of the corpus callosum ; and the gyrus 

 inf racallosus (where this exists) and the fornix and fimbria may also be considered to form 

 part of it. 2 



OLFACTORY LOBE. This lobe (fig. 112) lies at the ventral aspect of the 

 frontal lobe. It is rudimentary in man and other Primates, and in the seals, and is 

 lacking in Cetacea, but in all other mammals (osmatic mammals, Broca), and in 

 vertebrates generally, it is well developed and forms a distinct portion of the cerebral 

 hemisphere, enclosing an extension of the ventricular cavity. In some mammals 

 (e.g., horse) this extension remains throughout life in free communication with the 

 anterior horn of the lateral ventricle, in others (e.g., dog) the communication is lost. 

 In the human foetus of from two to four months, it appears as a hollow projection 

 of the fore-brain, but as the Avails of this projection thicken by the development 

 of nervous tissue within them, the cavity becomes gradually obliterated, and is 

 ultimately entirely occupied by a mass of neuroglia (central neuroglia of the olfactory 

 tract and bulb). Compare Vol. I., pp. 71 and 79. 



Anteriorly the olfactory lobe is connected with the olfactory cells of the olfactory 

 mucous membrane, posteriorly it is connected with the two extremities of the limbic 

 lobe. 



The olfactory lobe may be described (His) as composed of two parts or lobules, 

 an anterior and a posterior. The anterior olfactory lobule comprises (1) the olfactory 

 bulb, which rests on the cribriform plate of the ethmoid bone, and receives the fibres 

 of the olfactory nerves, which originate in the cells of the olfactory mucous 

 membrane, (2) the olfactory tract, which lies in the olfactory sulcus of the frontal 

 lobe, and posteriorly bifurcates into two roots, mesial and lateral, which diverge 

 as they pass backwards and enclose (3) a space, the trigonum olfaclorium, which is 

 also known as the middle or grey root of the tract, (4) the area of Broca (His), a 

 portion of grey matter lying between the mesial root and the peduncle of the 

 corpus callosum, and continuous with the commencement of the callosal gyrus. 

 This area is separated from the posterior part of the gyrus rectus of the frontal lobe 

 by an oblique sulcus (fissura serotina, His). 



The posterior olfactory lobule is marked off from the anterior by a curved fissure 

 (fissura prima, His). It is formed by the portion of brain cortex, which 

 appears on the surface at the anterior perforated space, and is bounded mesially 

 by the peduncle of the callosum (gyrus subcallosus of Zuckerkandl), anteriorly by the 

 fissure just referred to, whilst laterally it is continued into the vallecula Sylvii, 

 and is concealed by the temporal lobe which overlaps it. The continuation of the 

 lateral root of the olfactory tract courses lateral wards and backwards over the surface, 

 as it passes towards the anterior end of the hippocampal gyrus. 



The olfactory bulb (figs. 103, 112) is oval in shape and of a reddish-grey 

 colour when viewed from the ventral aspect. It is nearly a centimeter long and 

 about a third of this in width ; from its posterior extremity the olfactory tract 

 emerges. Its dorsal surface, which is in contact with the frontal lobe, is white and 

 is directly prolonged into the tract. This surface presents a longitudinal ridge 

 which fits into the anterior end of the sulcus olfactorius of the frontal lobe. 



The olfactory tract, sometimes erroneously spoken of as the olfactory nerve, is 

 a band of white matter, flattened on the ventral aspect but ridged along the dorsal 



1 Not to be confounded with the gyrus marglnalis on the mesial surface of the frontal lobe (see p. 148). 



z Zuckerkandl describes two bordering gyri, an outer and inner, the outer being formed by the dentate 

 gyrus, the gyrus supracallosus and the gyrus geniculi, and continued into the inner root of the olfactory 

 tract ; the inner by the fimbria, fornix, lamina septi lucidi. and gyrus subcallosus. 



