622 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



rhomboidal sinus : this may be seen in the 

 goose. The medulla oblongata, with the py- 

 ramidal and cerebellic fasciculi, present similar 

 characters to the same parts in the fishes ; the 

 corpora olivaria, and pons Varolii, are not yet 

 developed. 



The brain of birds is composed of a similar 

 number of parts as in the reptiles, but they are 

 more highly developed, and they are no longer 

 arranged in a longitudinal series as formerly, 

 but more on the top of each other. In the 

 sea-gull (Lams cyanorhynchus, Jig. 355), 

 snipe (Scolopax gaUinula), red start ( Mota- 



cilla ?), goldfinch ( Fringilla carduelis ), 



fowl ( Phasianus gallus, Jig. 356), pigeon 

 (Columba- ?), and hawk (Falco nisus), 

 the brain fills entirely the cranial cavity, this 

 cavity now corresponding exactly with the size 

 and form of the head. It is of increased re- 

 lative size compared with the body : in a pi- 

 geon, weighing according to Cams 3360 grains, 

 the brain weighed 37 grains ; the proportions 

 being as 100:9,100. 



On taking a review of its structure, we find 

 three principal portions, as heretofore, to oc- 

 cupy our attention, the conformation of each 

 being very uniform in the whole class : 1st, the 

 cerebral hemispheres ; 2d, the optic lobes ; 3d, 

 the cerebellum. 



1st. The cerebral hemispheres, or first cere- 

 bral mass, are large (fgs. 355, 356"', a, a), of 

 greater relative size than any other parts of the 

 brain, and vary but little in form ; in the embryo 



Fig. 355. 



Fig. 357. 



Brain and portion of spinal marrow of Lams 

 rhynchus (sea-gull), about natural size. 

 a, a, first cerebral mass or cerebral hemispheres. 

 b, b, second cerebral mass or optic lobes, c, third 

 cerebral mass or cerebellum, with its transverse 

 grooves, d, spinal cord, with its posterior longi- 

 tudinal fissure. 



chick on the sixteenth day, however, I found 

 them very little larger than the optic lobes 

 (fig. 357, a). In the sea-gull and snipe they 

 are of an oblong form, and larger posteriorly ; 

 in the hawk more round and short ; in the 

 red-start, goldfinch, pigeon, and embryo chick, 

 on the twentieth day, more lengthened in form 

 and covering quite the optic lobes (Jig. 357, (/). 

 In the ostrich they are also lengthened and 

 approach very much the form and characters 

 of the same parts in the lower Mammalia. 

 These hemispheres are united to each other by 

 a commissure (the anterior commissure); above 



Fiy. 356. Brain and portion of spinal cord of Pha- 

 sianu-s gallus (fowl), embryo state, age 16 days, slightly 

 magnified. 



o, a, first cerebral mass or cerebral hemispheres 

 of a triangular form, b, b, second cerebral mass 

 or optic lobes, touching at their inner borders, c, 

 third cerebral mass or cerebellum, small. </, spinal 

 cord, with its posterior longitudinal fissure, g, 

 olfactory nerves. 



Fig. 357. The same, age 20 days, slightly magnified. 



a, a, first cerebral mass or cerebral hemispheres, 

 oval in form. b, b, second cerebral mass or optic 

 lobes, widely separated from each other, c, third 

 cerebral mass or cerebellum, greatly increased in 

 developement and pushing asunder the optic lobes. 

 d, spinal cord, with its posterior longitudinal fissure. 

 g, olfactory nerves. 



this there is another one, which Meckel con- 

 siders as the first rudiment of the corpus cal- 

 losum * they contain cavities which are true 

 lateral ventiicles, and in which is a tubercle or 

 enlargement corresponding to the corpus stri- 

 atum of the Mammalia. From the anterior 

 part of this primary cerebral mass the olfactory 

 nerves arise (Jigs. 356, 357, g, g}, and pass 

 forwards to the cribriform plate of the ethmoid 

 bone ; their origin is marked by two distinct 

 enlargements, which are hollow and communi- 

 cate with the lateral ventricle. 



2d. The optic lobes, or second cerebral mass, 

 (Jig. 355, b), are of small size, and are more 

 widely separated from each other than in the 

 preceding classes, though they still are con- 

 nected by a medullary membrane correspond- 

 ing to the roof of the aqueduct of Sylvius. 

 In the embryo of the chick on the sixteenth day, 

 however, these parts, as before observed, are 

 nearly as large as the cerebral hemispheres, 

 their inner borders touching each other, as in 

 the reptiles and fishes (jig. 357, b} : at the twen- 

 tieth day they are widely separated (Jig. 356 //). 

 In the sea-gull (Jig. 355), snipe, hawk, red- 

 start, goldfinch, fowl, and pigeon, they are of 

 a rounded form, and situated immediately be- 

 neath the cerebral hemispheres. On cutting 

 into them in the sea-gull and pigeon, they are 

 found to contain a cavity, which, in the former, 

 is very small, in the latter larger, and con- 

 taining a solitary dark-coloured tubercle : 

 their cavities communicate with the third 

 ventricle. Between these optic lobes, and 

 immediately inferior to the cerebral hemi- 

 spheres, in the pigeon, there is situated a 

 pair of ganglia, of a flattened form (the ex- 

 istence of which has been before noticed in the 



* Archiv. fur Physiologic. 



