852 



COMPARATIVE ANATOMY 



the cortex in the case of painful sensibility and the entire series of pleas- 

 urable and painful qualities ; for the thalamic centers when isolated from 

 their cortical connections are found to be concerned mainly with affective 

 experience, and destructive lesions which involve the cortex alone do not 

 disturb the painful and affective qualities of sensation." 



Hypothalamus. That portion lying immediately beneath the thala- 

 mus. A small portion of the primitive neural tube to which the hemi- 

 spheres are attached has remained in a primitive state, not changing or 

 having any ingrowths or outgrowths. This unchanged portion is called 

 the pars optica hypothalami, and, as will be noticed by the table, is a 

 part of the end-brain and not the diencephalon. The hypothalamus is an 

 important correlation center for olfactory and various visual impulses, 

 including probably the sense of taste. 



Tuber cinereum is the gray eminence forming the ventral portion of 

 the hypothalamus. 



Infundibulum is a funnel-shaped extension of the third ventricle 

 passing through the hypothalamus to the end of the hypophysis 

 (pituitary body or gland which lies in the sella turcica). 



Mammillary bodies are a pair of eminences at the posterior end of 

 the tuber cinereum. These bodies are olfactory centers. 



Metathalamus. The posterior part of the thalamus consists of the 

 geniculate bodies. The lateral or external one is a visual center in the 

 thalamus and the medial or internal body is an auditory center. 

 Epithalamus. This is formed by the mem- 

 branous choroid plexus- which forms the roof of the 

 third ventricle, the habenula, and the stria medul- 

 laris, a fiber-tract which connects the olfactory cen- 

 ters of habenula and cerebral hemisphere. The 

 habenula itself is a center for the correlation of olfac- 

 tory sensory impulses with the various somatic 

 sensory centers of the dorsal part of the thalamus. 

 The pineal body, in a very few lower vertebrates, 

 is a sense organ, being called a "parietal eye" (Fig. 

 476). In the higher forms this sensory function has 

 been lost, though it is now supposed to be an organ 

 of internal secretion. 



Fig. 476. 



Anlage of the epiphysis 

 (pineal gland) and pari- 

 etal organ in the lizard 

 Iguana. A in a 9 day 

 embryo, and B in an 18 

 day embryo. Longitu- 

 dinal section, ep, epiphy- 

 sis ; pa, parietal organ ; 

 zw.h., wall of the ven- 

 tricle in the twixt-brain. 

 (After von Klinkow- 

 strom.) 



TELENCEPHALON 



In all comparative studies of animals one must 

 observe lower forms in order that the simpler ar- 

 rangement there found may furnish an understand- 

 ing of the more complex adjustment found in the 

 higher forms, as these latter, usually, possess parts 

 that the lower forms possess, plus something additional. 



In the study of the nervous system the dogfish is a good laboratory 



