NERVOUS SYSTEM 851 



well developed and is there called the subthalamus, which is often con- 

 fused with the hypothalamus. 



The dorsal portion of the thalamus is again divided into two por- 

 tions : 



(1) The primitive sensory reflex centers, principally in the medial 

 group of thalamic nuclei. 



(2) The regular cortical vestibule which forms the lateral nuclei. 

 These lateral nuclei are sometimes called the new thalamus (neothala- 

 mus) to distinguish them from all other portions of the thalamus, which 

 other portions are then called the old thalamus (palaeothalamus). 



In man the new thalamus makes up by far the greater portion of 

 that organ. This portion includes "the lateral, ventral, and posterior 

 nuclei (for general cutaneous and deep sensibility) receiving the spinal, 

 trigeminal, and medial lemnisci ; the lateral geniculate body and pulvinar 

 (visual sensibility) receiving the optic tracts; the medial geniculate body 

 (auditory sensibility) receiving the lateral or acoustic lemniscus." 



It will be noted that Professor Herrick, from whom this quotation 

 is taken, considers the two geniculate bodies as a part of the thalamus, 

 whereas our table calls them the metathalamus. The student will see 

 that all these parts are most intimately connected, and classification is 

 bound to be arbitrary no matter what pains may be taken to make such 

 classification as scientific as possible. 



All the lateral nuclei are connected with the cerebral cortex by im- 

 portant systems of fibers running both to and from the cerebral cortex. 

 The fibers themselves are called sensory projection fibers and all of them 

 pass through or near the internal capsule of the corpus striatum. 



While these lateral nuclei receive the impulses from the somatic 

 sensory fibers as well as the deeper sensibility impulses (such as touch, 

 temperature, pain, general proprioceptive sensibility, spatial localization, 

 etc., termed as a whole the somesthetic group), this latter group is prob- 

 ably separately represented in the thalamus although w r e have not yet the 

 evidence to demonstrate it. 



Each of the chief functional regions in the thalamus is connected 

 with a specific region in the cerebral cortex by its own projection fibers, 

 the tracts being known as radiations. For example, there are optic radia- 

 tions, auditory radiations, somesthetic radiations, etc. 



The old thalamus, which comprises the more medial thalamic cen- 

 ters found in lower forms, has little or no cerebral cortex, such as fish, and 

 seems to retain its function in higher vertebrates. In other words, some 

 "awareness" of what is going on is carried by these medial centers, so 

 that the cerebral cortex is not absolutely necessary for the animal to be 

 aware of its own action or reaction. This means that the cerebral cortex 

 is not necessary for all, though it undoubtedly is for most conscious 

 purposes. 



Professor Herrick says : "The thalamus can act independently of 



