SCIENCE. 



73 



SATURDAY, AUGUST 14, 18 



CONTRIBUTIONS TO ENCEPHALIC ANATOMY. 



BY E. C. SPITZKA, M.D. 



Having, through a piece of good fortune, come into 

 the possession of a living iguana, and thence obtained 

 the brain and cord in a perfectly fresh condition, I 

 was enabled to make a study for the first time of the 

 remarkable brain of this saurian. 



As regards the exterior of the encephalon, it pre- 

 sents nothing very different from that of any other 

 higher reptile. On a lateral view, however, it exhibits 

 a much acuter basilar incurvation, approximating to 

 the bird's brain in this respect. As in birds, also, the 

 optic nerves leave the skull directly on emerging from 

 the chiasm. It is remarked also that the optic lobes 

 are far larger than in any reptile or bird thus far ex- 

 amined by anatomists ; in fact, excluding the case of 

 the finny tribes, it may be said that the iguana pos- 

 sesses the largest optic lobes in the animal kingdom. 

 They are as massive in their grey and white tissues, 

 and nearly as voluminous as the cerebral hemispheres. 



The olfactory lobes and bulbs offer nothing special 

 for consideration. 



On a transverse section through the cerebral hemi- 

 spheres, I am able to identify the component parts of 

 the cornu ammonis of the mammalia. It appears 

 that the medial thin wall of the cerebral vesicle corre- 

 sponds, with its layer of closely packed pyramidal 

 nerve cells, to the stratum corporum nervorum arcto- 

 rum of Kuppfer, and it is indeed separated from the 

 cortical layer of the convexity, which I believe to cor- 

 respond, as far as the thin part extends, to the sigma 

 of the cornu. At the lower end of the thin-walled 

 vesicle, where a transition of nerve fibres from 

 the stratum corporum nervorum arctorum (?) takes 

 place, to the thalamus, and which therefore corre- 

 sponds to the fornix, there is an accumulation of 

 molecular nerve substance, projecting outwards into 

 the ventricular cavity. This may represent one of the 

 thalamic tubercles ; I regard it as much more proba- 

 ble, however, that it corresponds to the body of the 

 so-called fascia dentata. 



Now, in sections exhibiting the above features, I 

 find also another which is highly important, in so far 

 as it tends to overthrow another one of the dicta on 

 whose strength the sauropsidean and mammalian 

 brains are distinguished. Immediately underneath 

 the median longitudinal fissure, but over the third 

 ventricle, there passes a fasciculus of white fibres, 

 uniting the two hemispheres, and particularly that 

 portion of each which corresponds to the cornu am- 

 monis. This is unquestionably the corpus callosum, 

 whose first appearance in the embryo and the lower 

 mammalia we know to be intimately associated with 

 the development of the cornu ammonis. 



But it is when we reach the mesencephalon and 

 the region posterior to it, that we discover the most 

 remarkable features of this brain. 



As in some other saurians, the cerebellum instead 



of being curved backward, and constituting a cap 

 over a part of the lateral ventricle, as in the alligator 

 and chelonia, is bent forward, and bound to the 

 posterior face of the optic lobes by the arachnoid 

 filaments. On separating and drawing it backward, 

 thus making it correspond artificially in position with 

 the cerebellum of the alligator, we find that between 

 the optic lobes and the cerebellum there are two pairs 

 of tubercles. 



One of these pairs, which I have found as a con- 

 cealed mass in turtles, and as a very distinct elevation 

 in the alligator, ophidia and pseudopus, I was familiar 

 with, and I had no hesitation in describing it as the 

 post-optic ganglia corresponding to the posterior pair 

 of the corpora quadrigemina. The other was at first 

 new to me, but after a careful comparative study I 

 found that it was nothing but an unusually large, and 

 therefore more prominent representative of a gang- 

 lionic mass which I have noticed in fair development 

 in the turtle, and which is even represented in an 

 atrophic condition with the mammalia. As the pair 

 of tubercles in the iguana lies intermediate to the 

 optic and post-optic lobes, I propose for it the name 

 of inter-optic lobes. 



On a dorsal view these different parts lie about as 

 follows : In front are the massive optic lobes touching 

 each other broadly on the middle line, so that their 

 posterior margins form a continuous semi-lunar curve, 

 convex behind. Behind each optic lobe, and bulging 

 out somewhat, laterally, we have the smaller but dis- 

 tinct post-optic lobes, which fail to come in contact 

 in the median line, so that a shallow groove would 

 separate them, if it were not filled out by another 

 structure now to be described. 



If we imagine the median furrow separating the 

 optic lobes prolonged between the post-optic lobes, 

 and crowd two little pea-shaped eminences on each 

 side of this imaginary median line, so that the latter 

 are bounded in front by the optic lobes, on the out- 

 side by the post-optic lobes, and behind by the cere- 

 bellum, we will have the precise situation of the 

 inter-optic lobes. These eminences are not so re- 

 markable for their absolute size (their surface extent 

 being only half that of the optic lobes) as for the 

 distinctness of their demarcation. I have obtained 

 sections through their posterior third, in which these 

 bodies are shown to be absolutely free. 



Other sections further forward show that these 

 ganglia crop out of a specialized division of the cen- 

 tral tubular grey of the aqueduct, and that the visible 

 eminences do not represent the true extent of the 

 ganglia. 



The trochlearis nerves arise behind the inter-optic 

 lobes, and passing forwards and downwards, lie in the 

 furrow between the optic and post-optic lobes, as in 

 other reptiles. It is well known that in the mammalia 

 they pass down behind the post-optic lobes. I look 

 on this as an incidental and insignificant variation. 



The remainder of the isthmus shows nothing 

 especially noteworthy. The remarkable size of the 

 oculo-motor nuclei, and the gigantic dimensions of 

 their almost star-like multipolar nerve-cells, merits 

 mention, as well as the fact that in this animal the 

 nuclei of the third and fourth pairs constitute a com- 

 mon cell mass, unlike the relation in the mammalia, 

 and that the third and fourth pairs arise almost in the 



