320 NERVOUS SYSTEM. 



pillar: thus united, they wind upward, describing an arc, and, 

 as soon as they make their appearance within the ventricles, con- 

 stitute the body of the fornix. The superior crura, which are 

 comparatively slender, proceed from the upper end of the fornix, 

 wind upward, and then descend into the superior cornua of the 

 lateral ventricles, where they grow tapering, and at length end in 

 sharp-pointed extremities. Their anterior or concave edges, 

 which are thin, unattached, and somewhat uneven, have been 

 named the corpora Jimhriat a; their posterior borders are continu- 

 ous with the crura hippocampi : along their fimbriated edges run 

 the lateral processes of the plexus choroides. The middle part 

 or arch of the fornix is unattached ; but its superior or broad 

 part is contiguously applied behind to the thalami, and adheres 

 to them through the intervention of a fine membrane, investing 

 the vessels of the choroid plexus which are ramifying hereabouts, 

 named the velum hiterpositum : from the circumstance of the 

 back of the fornix being imprinted by these vessels with many 

 minute linear grooves, mostly running obliquely, this surface of it 

 is commonly described as the lyra,psalterium, or harp. 



Thalami. — Having divided and reflected the fornix, and turned 

 back the heads of the hippocampi, we bring into view the 

 thalami nervorum opticorum. These bodies are also said to be 

 in the lateral ventricles : more properly speaking, they form the 

 upper and back parts of those cavities. They are white and 

 conoid in form ; narrow and approximated, inferiorly, where they 

 lie between, and rather behind, the corpora striata ; broad and 

 directed opposite ways, superiorly; in front, they are opposed to 

 the fornix, which they in a measure support; and behind, they 

 contract into medullary bands — the tracfus optici, which turn 

 round the crura cerebri to the base of the brain. Upon these 

 bodies the separate portions of the plexus choroides unite into a 

 single vascular chord, which takes its course along the canal 

 between them, and makes its exit from the brain through the 

 fissure left between the posterior lobes of the cerebrum. The 

 thalami are firmer in their composition than the corpora striata : 

 like those bodies, however, they are cortical within, thinly striated 

 with medullary matter. 



Taenia. — In the groove between the thalamus and corpus stri- 

 atum, partly covered by the plexus choroides, runs a conspicuously 

 white, medullary band, designated the centrum semicirculare ge- 

 minum, vel feeuia semicircularis. 



Commissures. — The contiguous parts of the thalami, flattened 

 and closely applied, are united in one broad circular place by 

 pulpy cortical matter, which union is called their commissura 

 mollis. Immediately behind the fore part of the fornix, runs 



