is. 
The fibres of the delicate olfactory nerves are to be regarded as the axons of the 
olfactory cells of the olfactory mucous membrane. They enter the ventral surface of the 
olfactory bulb, and there each breaks up in an arborescent fashion into a tuft of terminal 
filaments. A thick dendrite from a mitral cell of the bulb passes down towards this 
terminal tuft, and, coming into contact with it, breaks up and terminates in a similar 
manner. In this way a large number of globular bodies, formed by the arborescent 
terminations of a mitral dendrite and of certain olfactory nerve-fibres, are formed. ‘These 
are the olfactory glomeruli of the bulb. The mitral cells lie deeper in the olfactory 
bulb. Each gives off several dendrites and one axon. Only one dendrite enters into the 
formation of a glomerulus, but several nerve-fibres may be connected with such a body. 
It thus happens that, through its dendrite, a mitral cell may stand in connexion with 
several olfactory nerve-fibres. The axon of the mitral cell passes upwards to the white 
matter of the bulb, enters this, and is conducted through the tract towards the cerebral 
cortex. 
546 THE NERVOUS SYSTEM. 
WHITE MEDULLARY CENTRE OF THE CEREBRAL HEMISPHERE. 
The white matter of the hemisphere which lies subjacent to the gray cortex is 
composed of medullated nerve-fibres, arranged in a very intricate manner. Accord- 
ing to the connexions which they establish these fibres may be classified into three 
distinct groups, viz. (1) commissural fibres; (2) association fibres; and (3) projec- 
tion fibres. 
Commissural Fibres.—These are fibres which link together portions of the 
gray cortex of opposite cerebral hemispheres. They are arranged in three ae 
forming three definite structures, viz. the corpus callosum, the anterior commissure 
and the psalterium or the hippocampal commissure. 
The corpus callosum has in a great measure been already studied (p. 528). As 
it enters each hemisphere, its fibres spread out in an extensive radiation (the radia- 
tion of the corpus callosum). It thus comes about that every part of the cerebral 
cortex, with the exception of the bulbus olfactorii and the under and fore part of 
the temporal lobe, is reached by the ecallosal fibres. But it should be clearly under- 
stood that all the regions of the cortex do not receive an equal proportion of fibres ; 
in other words, some cortical areas would appear to be more plentifully supplied 
than others. Another point of some importance consists in the fact that the callosal 
fibres do not, as a rule, connect together symmetrical portions of the gray cortex. 
As the fibres cross the mesial plane they become greatly scattered, so that most 
dissimilar parts of the cortex of opposite hemispheres come to be associated with 
each other. 
Kach callosal fibre arises in one hemisphere and ends by fine terminal arborisations 
in the cortex of the opposite hemisphere. It may arise in any one of three ways, viz. 
(1) as the axon of one of the cortical cells, either pyramidal or poly morphic ; (2) as the 
ee of a fibre of association ; (3) as the collateral of a fibre of projection. 
Many cases have been recorded in w hich, through congenital defect, the corpus 
callosum has not been developed. In the description of this structure on p-. 528 attention 
has been called to a layer of callosal fibres which sweep over the posterior and descending 
horns of the lateral ventricle, so as to form the immediate outer wall of the cavity. This 
layer is called the tapetum, and it has been stated that when the corpus callosum is 
absent the tapetum is found in a well-developed condition. Further, it has been asserted 
that in cases where the corpus callosum has been experimentally destroy ed the tapetum 
suffered no degeneration (Muratoff). Certain anatomists are, therefore, inclined to argue 
that the tapetum has little or no connexion with the corpus callosum. This assertion, 
however, cannot by any means be regarded as being proved. ‘There is a large amount of 
evidence on the other side. Thus, Mingazzini has seen a case of failure of the corpus 
callosum which was accompanied by a corresponding defect in the tapetum, whilst soften- 
ing of the splenium and the forceps major has been observed by Anton to be accompanied 
by a secondary degeneration of the tapetum. Further, the recent experimental evidence 
of Ferrier and Turner would appear to support the older view that the tapetum is associated 
in the closest manner with the corpus callosum. 
The anterior commissure (commissura anterior) is a structure supplemental to 
the corpus callosum. It connects together the two olfactory lobes, and also portions 
of opposite temporal lobes. It presents a cord-like appearance and is arranged in 
Po 
