FUNCTIONS OF THE SPINAL CORD. 705 



hemisphere. So in the Human embryo of the 6th week, we find a distinct 

 vesicle for the Thalami Optici, interposed between the vesicle of the Corpora 

 Quadrigemina and that which gives origin to the Cerebral Hemispheres; whilst 

 the Corpora Striata constitute the floor of the cavity or ventricle which exists 

 in the latter, this being as yet of comparatively small dimensions. Now, 

 as already pointed out, we may distinguish in the Medulla Oblongata and 

 Crura Cerebri, a sensory and motor tract, by the endowments of the nerves which 

 issue from them. The sensory tract maybe traced upwards, untilit almost entirely 

 spreads itself through the substance of the Thalamus. Moreover, the Optic 

 nerves, and the peduncles of the Olfactive, may be shown to have a distinct 

 connection with the Thalami ; the former by the direct passage of a portion of 

 their roots into these ganglia; and the latter through the medium of the Fornix. 

 Hence we may fairly regard the Thalami Optici &$ the chief focus of the Sensory 

 nerves, and more especially as the ganglionic centre of the nerves of common 

 sensation, which ascend to it from the Medulla Oblongata and Spinal Cord. 

 On the other hand, the Corpora Striata are implanted on the Motor tract of the 

 Crura Cerebri, which descend into the Pyramidal columns ; and their relation 

 to the fibres of which that tract is composed appears to be essentially the same 

 as that which the Thalami bear to the sensory tract. Upon the precise nature 

 of that relation, Anatomists are not agreed ; but there are several considerations 

 which render it probable that there is not that continuity between the fibres of 

 the Crura Cerebri, and those which radiate from the Thalami Optici and Corpora 

 Striata to the surface of the Hemispheres, which a superficial examination would 

 seem to indicate ; but that the fibres which ascend from the Crura Cerebri for 

 the most part, if not entirely, terminate in the vesicular substance of the former 

 bodies, and that the radiating fibres of the latter take a fresh departure from 

 them. 1 At any rate, as we shall see hereafter, there is a complete physiological 

 separation between the Cerebrum and the Sensory Ganglia upon which it is 

 superposed. 



731. The Thalami Optici, and the Corpora Striata, as is well known, are very 

 closely connected with each other by commissural fibres; and, if the preceding 

 account of their respective offices be correct, they may be regarded as having 

 much the same relation to each other as that which exists between the pos- 

 terior and anterior peaks of vesicular matter in the Spinal Cord f the latter 

 issuing motor impulses in respondence to sensations excited through the former. 

 They are also closely connected with other ganglionic masses in their neighbor- 

 hood, such as the Locus Niger, and the vesicular matter of the Tuber Annulare ; 

 which again are in close relation with the vesicular matter of the Medulla 

 Oblongata. Altogether it is very evident that a series of true ganglionic cen- 

 tres exists at the base of the Encephalon, and that these are really as distinct 

 from either the Cerebrum or Cerebellum as the latter are from each other; and 

 we have next to inquire, what functions are to be assigned to them. 



73$. The determination of these may seem to be the more difficult, as it is 

 impossible to make any satisfactory experiments upon the ganglionic centres in 

 question by isolating them completely from the Cerebral Hemispheres above, 

 and from the Medulla Oblongata and Spinal Cord below. But the evidence 

 derived from Comparative Anatomy appears to be in this case particularly 

 clear; and, rightly considered, affords us nearly all the information we 

 require. In the series of "experiments prepared for us by nature," which 

 is presented to us in the descending scale of Animal life, we witness the effects 



1 See especially Messrs. Todcl and Bowman's "Physiological Anatomy," p. 251, Am. Ed., 

 and Prof. Kolliker's " Mikroskopische Anatomic," band ii. g 118. 



2 This was first pointed out by Messrs. Todd and Bowman in their "Physiological 

 Anatomy," p. 308, Am. Ed. 



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