THE CEREBRUM, AND ITS FUNCTIONS. 691 







parts of the apposite, hemispheres are connected by the transverse fibres of the corpus 

 callosum. The cortex of the cerebrum is connected with that of the cerebellum by 

 fibres passing directly from one to the other. 



The cr/tfs sh'lutinn consists of two nuclei, a smaller one the caudate or intra- 

 ventricular nucleus (CN, Fig. 244) and a larger or extraventricular one, named 

 the lenticular nucleus (LN). The caudate nucleus is essentially connected with the 

 olfactory lobe and bulb by connecting fibres (O), and is consequently small in man as 

 compared with many animals. 



The fibres forming the first link of the projection system, and connected with the 

 lenticular nucleus, are 1. Those proceeding from the internal surface of the whole 

 length of the cerebral hemisphere, but chiefly from the frontal lobes. 2. The stria 

 cornea (SC) proceeding from the apex of the temporal lobe to the anterior region of 

 the corpus striatum. 3. Then in addition there are the fibres entering the optic 

 thalnnms namely, a Those arising from the frontal lobe and passing straight 

 back between the caudate and lenticular nuclei of the corpus striatum (a, Fig 244). 

 b. Those proceeding from the walls of the Sylvian fissure and from the temporal 

 lobes b (inferior peduncle of optic thalamus). c. Fibres from the striatum zonale ; 

 and lastly, d, Those proceeding from the cortex of the gyrus fornicatus, constituting 

 the ascending pillar of the fornix (superior peduncle of optic thalamus). The fornix 

 itself is the projection fasciculus of the cortex passing to the anterior eminence of 

 the optic, thalami. 



The deeper-lying part of the optic thalamus receiving the corona radiata contains 

 many cells, and constitutes the red nucleus of the tegmentum, RN. This red nucleus 

 is the first internode of a tract that originates in the cortex of the cerebrum in a fan- 

 like form, and stretches to the cortex of the cerebellum. The processus a cerebello 

 ad cerebrum is a free segment of this connecting tract between the two cortical sub- 

 stances. Beneath the above-mentioned fan is a second more slender fan of the corona 

 radiata, which is applied to the substance of Soemmerring. Posteriorly the optic 

 thalami are connected with the optic tracts. The optic tracts are also connected just 

 as if they were a portion of the corona radiata with the brachium of the inferior 

 corpus quadngeminum. The optic tracts and nerves, therefore, are not peripheric 

 nerves, but the retina, like the olfactory bulb, is, so to speak, an outlying portion of 

 the cortex cerebri. 



The second link of the projection system is formed by the system of the crus cerebri, 

 which is divided into two layers with some intermediate gray substance. The in- 

 ferior or anterior layer, named the crusta, proceeds from the ^corpus striatum ; the 

 superior or posterior layer, named the tegmentum, chiefly from the optic thalamus. 

 In addition to the fibres proceeding from the two nuclei of the corpus striatum, the 

 crusta contains others derived from the substantia nigra of the ventricles, fibres 

 (shown in dotted outline) which come directly from the occipital and temporal lobes. 

 The size of the cerebral hemispheres, of the lenticular nucleus, and of the crusta 

 cerebri, are all proportionate to one another, but there is no relation between the size 

 of the crusta and of the tegmentum. On the contrary, the tegmentum of the crus with 

 its ganglia, especially the corpora quadrigemina, and corpora geniculata interna, are 

 more largely developed in propertion to the smallness of the cerebral lobes. The teg- 

 mental fibres proceeding from the optic thalamus are reinforced by others from the 

 posterior commissure the anterior and posterior corpus genieulatum, the cortex cere- 

 belli, and the pineal gland, those from the latter source decussating with those of the 

 opposite side. The innermost fibres of the crusta, after decussating in the middle 

 line, terminate peripherically in the central gray substance of the ventricle surround- 

 ing the aquffiductus iSylvii in the nates, penetrating at this point into the nucleus 

 common to the third and fourth nerves. The most external fasciculus of the crusta 

 passes through the decussation of the pyramids of the medulla oblongata, into the 

 posterior column of the spinal cord. 



The tegmentum is characterized by its connection with the optic tracts and fifth 

 pair of nerves on the one hand, and with the motor columns of the spinal cord on 

 the other, forming a region where sensory impressions are reflected as motor impulses. 

 The fact that the ganglia of the crusta have no connection with sensorial peripheric 

 expansions, suggests that they owe their power of exciting motor impulses to another 

 source namely, to the conditions of excitation of the cerebral lobes. 



The organization of the crusta is completed near the anterior part of the thalamus. 

 That of the tegmentum just below the corpora quadrigemina, the lowest of its gan- 

 glia of origin. 



A large group of the radiating fibres proceeding from the occipital lobes converge 

 to the ganglia of origin of the tractus opticus i. e., the external and internal corpora 

 geniculata, and the corpora quadrigemina, forming a remarkably thick medullary 



