NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



621 



cupy our attention, the olfactory tubercles, 

 situated most anteriorly, the optic lobes, situated 

 posteriorly to these, and the cerebellum. 



1st. The olfactory tubercles, or first cere- 

 bral muss (figs. 352, 353, a, a, ), now be- 

 come obviously the cerebral hemispheres, are 

 of an increased proportional size, are com- 

 mencing to cover the tubercula quadrige- 

 mina, and contain a cavity which was first 

 developed in the Plagiostome fishes ; they are 

 very various as to form. Amongst the Am- 

 phibia, in the triton they are elongated and 

 oblong; in the frog, more oval they are united 

 at their anterior parts by a commissure, but 

 posteriorly they are separated. Amongst the 

 true Reptilia, in the viper and ringed snake 

 they are of a rounded form, and extended late- 

 rally ; in the lizard and turtle they are oval 

 (figs. 353, 354, , a) ; in the crocodile they 

 are more extended laterally. On cutting into 

 them, in the turtle there is found an oblong 

 tubercle analogous to the corpus striatum (fig. 



353, &*), on the inner side of which is a plexus 

 choroides. From the anterior part of these 

 hemispheres in the different animals mentioned, 

 the olfactory nerves arise, and run forwards to 

 the cribriform plate of the ethmoid bone, on 

 the upper surface of which, in the viper and 

 lizard, they form a bulbous enlargement (fig- 



354, g): in the turtle this is wanting, but at 

 their origin they form a large round hollow 

 swelling, situated immediately anterior to the 

 cerebral hemispheres, and communicating with 

 the cavities in their interior (fig. 353, i, k, /). 



Fig. 354. 



Brain and portion of spinal cord of La- 

 certa viridis (lizard), sliglttly magnified. 



a, first cerebral mass or cerebral 

 hemispheres, b, second cerebral mass 

 or optic lobes, c, third cerebral mass 

 or cerebellum, d, spinal cord, with 

 its posterior longitudinal fissure, e, 

 fourth ventricle, f, Pineal gland, g, 

 olfactory nerves, with their bulbous 

 enlargements. 



2d. The optic lobes, or second cerebral mass 

 (figs. 353, 354, b, b, 6), are of small size, 

 and are more solid than the same parts in the 

 fishes, the internal cavity being smaller : we 

 thus see them gradually approaching to the 

 form and character of the tubercula quadrige- 

 mina of the Mammalia, and of man. In the 

 triton, frog viper, ringed snake, lizard f^g.354), 

 and turtle (fig- 353); they are of a rounded 

 form, and situated on a plane immediately pos- 

 terior to the cerebral hemispheres. In all these 

 species there is found also, immediately anterior 

 to them, and partly covered by the cerebral 

 hemispheres, a pair of small ganglia, analogous 

 to the optic thalami of the human brain, on the 

 superior surface of which was situated the pineal 

 gland (fig. 354). These different eminences 

 give origin to the fibres of the optic nerves. 



3d. The third cerebral mass, or cerebellum 

 (figs. 353, 354, c), presents some inte- 

 resting grades of developement in these two 



classes of animals. In all of them it is small, 

 in most of them extremely small, and covers 

 in the fourth ventricle in a similar manner to 

 what has been described in the fishes. In 

 the triton and frog it consists of a thin 

 transverse band of medullary matter, pre- 

 cisely analogous to the cerebellum of the lam- 

 prey, and, as in that animal, leaving the fourth 

 ventricle quite open and exposed : in the viper 

 and lizard (fig. 353) it presents a similar ap- 

 pearance, but the band of medullary matter is 

 rather thicker ; in the turtle (fig. 352) it con- 

 sists of a tongue-shaped lobe, very similar to 

 the cerebellum of the cod : there are very dis- 

 tinct lateral appendages, the rudiments of 

 which we first observed in the Plagiostome 

 fishes, and which we shall trace in the suc- 

 ceeding classes to increased degrees of deve- 

 lopement : these lateral appendages are found 

 also in the crocodile ; they lead us, by strict 

 analogies, to the cerebellum of the birds. 



On reviewing these statements of the ner- 

 vous system of the Reptilia, we observe that 

 the equality and honzontality of the brain and 

 spinal marrow again claim our attention as 

 marks of low organization. Still, however, 

 the preponderance of the spinal marrow over 

 the brain is less, while the weight of this latter, 

 compared with the body, is greater. The first 

 cerebral mass has increased in size; cavities 

 are developed in its interior, and it is united 

 into two portions, which are divided by a com- 

 missure. The second cerebral mass, or tuber- 

 cula quadrigemma, has decreased in size, and 

 the cavities are much smaller. The third cere- 

 bral mass, or cerebellum, is in the lower Rep- 

 tilia imperfectly developed, but in the higher 

 ones it is of some size, and marked by ex- 

 ternal striae. 



4. AVES. In the class Aves, or birds, the 

 nervous centre has acquired a high degree of 

 developement in all its parts, but particularly 

 as regards the cerebellum, and the different 

 portions composing the cerebral mass are ar- 

 ranged more above and less behind each other. 

 The spinal cord (fig. 355, d) is of less re- 

 lative size, and of less extent, than in the fishes 

 and reptiles, but it still is traversed by an 

 anterior and posterior longitudinal fissure, and 

 still contains a central canal. In a pigeon 

 weighing (according to Cams) eight ounces, 

 (360 grains) the brain weighed 37 grains, and 

 the spinal marrow only 11 grains, the propor- 

 tion being as 100:30. We thus observe that 

 the brain now preponderates in size over the 

 spinal cord for the first time; this at once 

 marks its increased developement. Where the 

 nerves supplying the anterior and posterior ex- 

 tremities are given off, the spinal cord presents 

 distinct enlargements, the inferior of which is 

 the largest, and is placed in the sacrum : this 

 may be considered as the termination of the 

 spinal cord, for Carus considers that portion 

 passing through the coccygeal vertebra to be 

 only a large terminal filament. A canal passes 

 through the whole extent of the spinal cord, 

 which, at the inferior enlargement, forms a 

 large and remarkable excavation, called the 



