!'- General Consideration of Function [CHAP. 



in the anosmatic brain, and that its development varies directly with the development of 

 the olfactory apparatus : he also mentions that the anterior commissure and fornix are 

 developed proportionately with the fascia dentata. 



S. Ramon y Cajal from a histological study of the brains of the dog, cat, guinea-pig, 

 rabbit, rat, and mouse, as well as that of the human being, arrives at the following 

 conclusions. " 1. Wir miissen als secundare Riechcentren alle diejenigen betrachten, welche 

 unzweifelhaft Fasern der ausseren, mittleren oder oberen Wurzeln aufnehmen. Diese Centren 

 besitzen die gleiche Structur ; es sind die Rinde des Lobulus olfactivus (Pedunculus bulbaris), 

 die frontale, unter der ausseren Wurzel liegende, und die aussere Gegend der Sphenoidalrinde. 

 2. Der Focus spheno-occipitalis, das Subiculum, der Focus prasubicularis und das Arnmonshorn 

 scheinen keine directen Olfactoriusfasern zu besitzen. Vielleicht stellen sie tertiare Riech- 

 centren dar. 3. Die Amygdala, das Septum pellucidum, die limbischen Windungen, und die 

 Zwischenhemispharenrinde, die Striae supracallosae, die prachiasmatische Fissurrinde, etc., 

 entbehren anscheinend directer olfactiver Verbindungen 1 ." 



Summarising the deductions drawn from studies in comparative anatomy and applying 

 them to the human brain we gather that the lobus pyriformis must almost certainly be 

 regarded as the chief olfactory centre, but opinions are divided concerning the functional 

 importance of other parts of the cortex, that is, the cornu ammonis, gyrus fornicatus, and 

 other subsidiary structures. 



Points in the Development of tie Olfactory Lobe. 



The development of the various parts of the cerebrum concerned with the olfactory 

 sense has been studied or described with infinite thoroughness, by His, Sir William Turner, 

 Gustav Retzius, Ferrier, Edinger, Elliot Smith, and others, and the results of these researches 

 are so well-known that it is unnecessary for me to discuss them ; I will mention only 

 that to the student of localisation, points of special interest are the clearness with which 

 it may be seen that in the foetal brain the olfactory lobe has connections with the lobulus 

 pyriformis, and particularly with the gyrus circumambiens of Retzius ; and in what a definite 

 manner the fissura rhinica divides the posterior olfactory from the temporal lobe. It is also 

 interesting to know that the gyrus subcallosus of Zuckerkandl is a more prominent object 

 in the foetal than in the adult brain. 



Added to these anatomical researches we also have the advantage of referring to the 

 histological investigations of Flechsig and others, who have aimed at recording the period 

 and manner in which the nerve fibres derived from the olfactory roots become developed. 



Although in his original publications Flechsig was responsible for the statement that the 

 cortical fibres of the central convolutions were the first to acquire a myelinic investment, 

 and that the olfactory fibres came second, in a more recent paper giving the results of 



1 " 1. As secondary olfactory centres we must regard all those parts which undoubtedly receive fibres from the 

 external, middle, or upper roots. These centres possess a like structure ; they are the cortex of the lobulus olfactivus 

 (pedunculus bulbaris), the frontal cortex underlying the external root and the external part of the sphenoidal cortex. 

 (By tin' latter the Joints pyriformis is infant.) 



'"2. The focus spheuo-occipitalis (/if lower animals), the subiculum, the focus presubicularis, and the cornu ammonis 

 seem to receive no direct olfactory fibres. Perhaps they represent tertiary centres of smell. 



" 3. The amygdala, the septum pellucidum, the limbic gyri, and the inter-hemispheric cortex, the striae supra- 

 callosae, the prechiasmic fissural cortex, etc., are apparently deprived of direct olfactory associations." 



