PARTS DERIVED FROM THE FORE-BRAIN 911 



The two parts of the pituitary body are as distinct in structure as they are in embryonic 

 origin. The posthypophysis consists of a mass of nerve cells, neuroglia, connective tissue, and 

 bloodvessels; the structure of the prehypophysis is distinctly glandular, resembling that of the 

 parathyroid bodies. It is surmised that the latter is the functional part of the pituitary bodv 

 concerned with the internal secretions, and usually involved in the pathological form of giantism 

 called acromegaly. 



The lamina terminalis or terma (lamina terminalis s. cinereci) (Fig. 664) is a 

 thin, easily torn lamina between the optic chiasm and the anterior commissure, 

 limited laterally by the closely approximated cerebral hemispheres and con- 

 stituting the primitive, undifferentiated cephalic boundary of the original neural 

 tube. 



The Optic Tract and its Central Connections. -In the section on the development 

 of the brain it was learned that the optic nerve is not a peripheral nerve; it is 

 rather a central brain tract extruded from the neural tube. Evidence is at hand 

 that in ancestral vertebrates the general cutaneous sensor system was also capable 

 of light perception. With the recession of the neural tube from the surface and 

 in company with the morphological differentiation of the head end, a light- 

 perceiving pair of organs arose as a special development. The distal end of the 

 optic brain vesicle becomes the retina, in structure like the brain wall, whose cell 

 axones carry afferent impulses to the brain. Although the optic fibres enter the 

 ventral wall of the brain, the retina is originally derived from the dorsolateral 

 (sensor) wall of the second neuromere (Fig. 627). The parietal organs, also 

 light-perceiving, likewise developed as paired dorsal buds farther caudad, eventu- 

 ally to atrophy, as the more frontal optic organs better subserved the purposes 

 of the organism. 



The remarkable and as yet unexplained fact regarding the optic apparatus 

 is that the afferent fibres from the retinal cells pass into the ventral wall to cross 

 to the opposite side, forming a decussation which is total, or nearly so, in verte- 

 brates below the mammals; the more laterally placed the eyes are the more nearly 

 total is the decussation. 1 



Although the optic vesicle is a diverticulum of the fore-brain in its cephalic 

 portion, the optic tract in its central connections becomes intimately related with 

 the external geniculate body and pulvinar, with the superior quadrigeminal body 

 of the mid-brain, and with the occipital cortex of the cerebrum. Some of these 

 central structures are way-stations in reflex paths; the occipital cortex alone is 

 the actual visual centre, though visual perceptions are here brought into associa- 

 tion with tactile, auditory, and other impulses. 



Optic Chiasm. From the retina of each eye the so-called optic nerves-ron verge to 

 partially decussate at the base of the brain to form the optic chiasm, a white quad- 

 rangular plate which presses in the primitive central gray floor of the third ven- 

 tricle, as previously described. Approximately one-tfoird of the fibres of each 

 optic nerve do not cross to the opposite side. The optic chiasm is further re- 

 enforced by the infracommissure (of Gudden) and other lesser fibre tracts (com- 

 missura superior [Meynerti] and commissura ansota [Kolliker]). The fibres in 

 the chiasm are so complexly interwoven that only through exhaustive experimental, 

 developmental, and pathological studies has it been possible to understand its 

 structure. Broadly stated, the fibres from the medial (or nasal) halves of the retinae 

 decussate in toto. while those from the lateral (or temporal) halves do not cross. 

 Leaving the optic chiasm, the crossed medial and uncrossed lateral fibres form 

 the slightly flattened optic tracts coursing caudolaterad. embracing the crura 

 cerebri and dividing in the neighborhood of the lateral geniculate body into two 

 "roots." a mesal and a lateral root. The mesal root is in reality not a part of the 



1 Possibly the reflex contraction of the muscles on one side of the body in the ancestral vertebrate followed 

 the perception of a menacing object by the eye of the opposite side; hence the advantage of a decussation. 



