AVES. 



299 



are equally obvious in the brains of reptiles. 

 They are, however, differently disposed in birds ; 

 the optic lobes, which in reptiles intervene and 

 are visible between the cerebrum and cerebel- 

 lum, being in birds displaced, as it were, by 

 the hemisphere and cerebellum coming into 

 close contact, so that the optic lobes are pushed 

 downwards and to one side. The transverse 

 convolutions of the cerebellum at once distin- 

 guish, however, the brain of a bird from that 

 of any reptile and most fishes ; but it is a curi- 

 ous fact that the cerebellum in the sharks is 

 similarly composed of a vermiform process only, 

 transversely folded or convoluted. 



The cerebral hemispheres sometimes present 

 the form of a flattened oval, as in the Parrot 

 tribe, but in general are of a convex cordiform 

 shape, with the apex directed forward. 



Fig. 135. The optic lobes (b, 



jig. 135) are rounded 

 tubercles, situated be- 

 low and behind the 

 hemispheres, in the la- 

 teral interspace between 

 these and the cerebel- 

 lum. 



The cerebellum is 



Base of thTbrain of a Composed of the middle 

 Pigeon. *" e on 'y> an " 1S OI a 



compressed arched form. 



The medulla oblongata presents neither a 

 tuber annulare nor corpora olivaria or pyrami- 

 dalia, but is a large uniform tract situated be- 

 tween and behind the optic lobes. 



On the lower part of the side of each cere- 

 bral hemisphere there is a depression which 

 corresponds to the fissura magna Sylvii, and is 

 the only appearance which the hemispheres 

 present of a division into lobes. Elsewhere 

 there are no traces of convolutions, the cere- 

 brum in this respect resembling that of Rep- 

 tiles and Fishes, and some of the least intel- 

 ligent orders of Mammalia, as the Rodentia, 

 Marsupiata, and Edentata. The optic lobes 

 are also devoid of the transverse fissure which 

 bisects the optic lobes of mammalia. 



The cerebellum is marked by close and 

 transverse anfractuosities, such as characterize 

 the corresponding portion of the cerebellum in 

 mammalia, called the vermiform process. 



Fig. 136. When the cerebral 



hemispheres are divari- 

 cated from each other, 

 (Jig. 136J they are 

 seen to be disunited 

 through the whole of 

 their vertical extent, 

 and to be joined only 

 by the round anterior 

 commissure of the 

 Brain of a' Pigeon. brain (k,jig. 136.) In 

 fact both the corpus callosum and fornix are 

 wanting ; or at most a rudiment only of the 

 latter part can be perceived in the brains of 

 some birds, as the Eagles, Vultures, and Parrots. 

 The mesial surfaces of the hemispheres, which 

 are in contact with each other, present a few 

 striae which diverge from the commissure. 

 These surfaces are composed of an extremely 



thin layer of medullary substance, (g y ) forming 

 the internal parietes of the ventricle, and ex- 

 tended outwardly over the corpus striatum (i.) 

 This body is of very great size in birds, consti- 

 tuting of itself almost the entire substance of 

 the hemisphere, projecting into the ventricle, 

 (h,) not only from below, but from the anterior 

 and outer sides of the cavity, and being covered 

 by a smooth layer or fold of medullary matter, 

 (ft) which increases in thickness anteriorly. 

 The ventricle does not extend below the corpus 

 striatum to form an inferior horn; and, as in 

 most mammalia there is no extension of the 

 cavity backwards to form a posterior horn, 

 there is consequently no cornu ammonia. The 

 vessel forming the plexus choroides penetrates 

 the ventricle beneath the posterior part of the 

 thin internal wall, and the lateral ventricles 

 communicate together there, and with the third 

 ventricle. They are continued anteriorly to the 

 root of the olfactory nerve, which is itself a 

 continuation of the apex of the hemisphere. 



Just above the orifice of communication there 

 is a smooth flattened projection, rounded exter- 

 nally, which advances into the ventricle from the 

 internal wall ; this is a rudiment of the fornix. 



The round anterior commissure (k) is pro- 

 longed on either side into the substance of the 

 hemispheres, as in man and quadrupeds. 



The optic thalami (I) are of small size, and 

 not united by a soft commissure: between them 

 is the cavity called third ventricle (m) ; and 

 above and behind they give off the peduncles 

 of the pineal gland. This body does not hang 

 freely suspended by the pedicles, but seems to 

 form a rounded and thickened anterior border 

 of the valvulaVieussenii or lamelliform commis- 

 sure of the optic lobes. Carus describes the 

 pineal gland as adhering firmly to the conflu- 

 ence of the great veins situated at the anterior 

 orifice of the aqueduct of Sylvius. In Pigeons 

 he states that it is composed of many segments, 

 but that in general it is of a simple and conical 

 form ; the figure which he gives of it, from the 

 Turkey, exhibits a pyriform shape.* The valve 

 which closes the upper part of the passage 

 from the third to the fourth ventricle, is a thin 

 lamella of great width, in consequence of the 

 distance to which the optic lobes are sepa- 

 rated from one another. Anteriorly the third 

 ventricle communicates with the infundibulum. 



The fourth ventricle (n) resembles that in 

 the brain in mammalia, but is of less width ; 

 its floor is indented with the longitudinal fissure 

 called calamus scriptorius. 



Besides the cavities or ventricles above men- 

 tioned, there are also two others situated in the 

 optic lobes (o), or bigeminal bodies, each of 

 which, when laid open, is seen to be occupied 

 by a convex body (p) projecting from the 

 posterior and internal side of the lobe; these 

 ventricles communicate with the others in the 

 aqueduct of Sylvius. 



As there is no transverse furrow in the optic 

 lobes, they cannot be distinguished into the 

 protuberances called ' nates 7 and ' testes ' in 



* Anat. Comparcc, nouv. cd. i. p. 88, pi. xv. 



fig. 6. 



