700 



NERVOUS SYSTEM. (NERVOUS CEWTHES. THE ENCEPHALON.) 



however, do not form any intricate interlace- 

 ment as in ganglia, but are collected into bun- 

 dles of very variable size ; the largest being 

 placed at the inferior part of the body, the 

 smallest towards the hemispheres. The free 

 or ventricular portion of the corpus striatum 

 contains comparatively few fibres. When a 

 portion of the striated part of the body is 

 examined under the microscope, the nerve- 

 fibres, of which the bundles are composed, ap- 

 pear to be reduced to their smallest size, and 

 to be very compactly applied to each other, 

 so that they transmit very little light, and 

 therefore put on the appearance of dark cylin- 

 ders. It is only by very high powers that we 

 can discover their fibrous structure. In many 

 of the bundles the fibres appear to terminate 

 at one extremity as if by forming an adhesion 

 around a large vesicle, faint indications of the 

 nucleus and nucleolus of which may be some- 

 times seen through the surrounding fibres. The 

 appearance which the fibrous cylinders which 

 exhibit this structure present calls to mind 

 very strongly the representation of the nucleus 

 of a comet with its tail. And this peculiarity 

 of structure may be adduced as an argument 

 that many if not the greater number of the 

 fibres of the striated body form intimate connec- 

 tions with the elements of its vesicular matter. 



Optic tlialami. In the internal concave sur- 

 face of each corpus striatum, the optic thala- 

 mus is placed. The latter body is therefore 

 posterior and internal to the former. The 

 lighter colour of the optic thalamus distin- 

 guishes it at once from the corpus striatum. 

 The optic thalami come into close relation to 

 each other by their inner surfaces, which form 

 the lateral boundaries of the third ventricle. 



Each optic thalamus, like the corpus stria- 

 tum, presents a free and an attached portion. 

 The former projects into the ventricle the 

 intra-ventricular portion ; the latter adheres to 

 the inner side of the corpus striatum and to 

 the mass of the hemisphere, and posteriorly, to 

 the olivary columns, the quadngeminal tuber- 

 cles, and the processus cerebelli. The supe- 

 rior surface is free and forms part of the floor 

 of the lateral ventricle ; the internal surface is 

 likewise free and forms the lateral wall of the 

 third ventricle, being, however, interrupted in 

 a very small space by the adhesion of the soft 

 commissure. A portion of its external and 

 posterior surface is also free, and projects back- 

 wards and outwards into the inferior horn of 

 the lateral ventricle, presenting a pointed ex- 

 tremity in that situation. These free surfaces 

 are smooth and moist, being covered by the 

 membrane of the ventricles. The velum in- 

 terpositum, which again is overlapped by the 

 fornix, rests upon the superior surface of the 

 optic thalamus. 



The optic thalami are placed obliquely, so 

 that they are nearer each other at their anterior 

 than at their posterior extremities. Each mea- 

 sures about an inch and a half in length, nine 

 to ten lines in height, and about eight lines in 

 breadth. In colour they are very much lighter 

 than the striated bodies, and they appear to be 



covered with a delicate layer of fibrous matter. 

 A band of fibrous matter passes along the inner 

 surface of each from behind forwards, which 

 posteriorly is connected to the pineal gland, 

 and forms, with its fellow, the peduncles of 

 that body. 



Beneath the posterior free extremity of the 

 thalamus, situated in the angle between that 

 body and the superior surface of the crus, we 

 find a small rounded eminence of a darkish 

 grey colour perforated by very numerous fora- 

 mina for the transmission of bloodvessels. 

 This is the corpus geniculatum internum. 

 Lower down and more external and anterior, 

 there is another similar body, somewhat smaller 

 and less dark, the corpus geniculutum externum. 

 Both of these bodies are connected with the 

 quadrigeminal tubercles. A band of fibrous 

 matter passes from the testes to the external 

 geniculate body, and from the nates to the in- 

 ternal one. 



In point of structure the thalamus resembles 

 a ganglion much more closely than does the 

 corpus striatum. A light reddish grey is the 

 colour of the surface when cut into ; it has been 

 not inappropriately compared to that of coffee 

 mixed with a good deal of milk (cafe au lait). 

 When thin sections are examined, they are found 

 to consist of very numerous fibres interlacing 

 freely, with nerve vesicles occupying their in- 

 tervals. The fibres are not collected into bun- 

 dles as in the corpus striatum, nor do they take 

 a radiating course in the thalamus. The reti- 

 culation which they form is very like that in 

 the ganglia on the posterior spinal roots. 



The fibres of the optic thalami, inasmuch as 

 they are very numerous, have extensive con- 

 nections. Along its ventricular surface they 

 are evidently continuous with those of the he- 

 misphere, which appears to radiate from it to 

 the grey matter of the convolutions. Posteriorly 

 the fibres of the processus cerebelli ad testes 

 and those of the olivary columns pass into it. 

 The anterior pillars of the fornix are connected 

 with it in front, and derive from it some nervous 

 fibres ; and below and within, a cylinder of 

 fibres emerge from it to the mamillary bodies. 

 Thus the optic thalami are connected with the 

 hemispheres on one hand, with the olivary co- 

 lumns and with the cerebellum on the other 

 hand. The quadrigeminal tubercles placed 

 upon the processus cerebelli may have some 

 connection with them through the latter bundles 

 of fibres. Although these bodies have been 

 viewed as having a special connection with the 

 optic nerves, it does not appear that those 

 nerves have any relation to them but through 

 the geniculate bodies or the quadrigeminal 

 tubercles. It is important to bear in mind 

 respecting the optic thalami that they are di- 

 rectly continuous with the superior portion of 

 the crus cerebri, so that in viewing a vertical 

 section of the encephalon we see no line of 

 demarcation between. The thalamus grows, 

 as it were, from the superior extremity of the 

 crus ; it is recognised from the latter by its 

 swelling into an ovoidal mass. It is empha- 

 tically, as Willis long ago expressed it, an 



