790 TEE NERVES. 



CHAPTEE IV. 



THE NERVOUS SYSTEM IN BIRDS. 



PROTECTIVE PARTS OF THE CEREBRO-SPINAL Axis. The protective parts of the 

 nervous centres are the same in all vertebrate animals ; consequently, there is nothing 

 to remark regarding those of Birds. The envelopes or meninges are three in number, 

 and disposed as in mammals. 



" The falx cerebri is found in birds ; in the Turkey it has the form of the segment of 

 a circle, and extends from the middle of the interval of the openings for the olfactory 

 nerves to the tentorium cerebelli. The falx cerebelli is absent ; the tentorium is small 

 and sustained by a bony plate, and there are, in addition, two {articular folds, one on 

 each side, that separate the hemispheres from the tubercula quadrigetnina." Cuvier. 

 Owing to the absence of the falx cerebelli, the meninges of birds are closer together 

 than those of Solipeds or Man. According to Leydig, the falx cerebri is partially 

 ossified in birds. 



SPINAL CORD. In Birds, the spinal cord is perforated by a central canal, and also 

 offers, as in mammals, two enlargements- a cervico-dorsal and lumbar. It is prolonged 

 into the coccygeal vertebrae, and thus furnishes another proof against the assertion of 

 certain naturalists, who desire to establish a relation between the length of the spinal 

 cord posteriorly, and the development of the coccygeal region. The two fasciculi of the 

 medullary axis are separated from one another at the lumbar enlargement, and after- 

 wards join in the sacral region. Between them is an elliptical space, the rhomboidal 

 sinus, which is filled by transparent gelatinous connective substance a kind of efflor- 

 escence of the ependymis of the central canal. 



ENCEPHALON. In a medium-sized Fowl, the encephalon weighs about 2J drams, and 

 comprises the three portions present in the mammalia. 



The medulla oblmgata is not divided into two sections by the pons Varolii, which is 

 absent in birds; the crura cerebelli are immediately connected with the corpora 

 restiformia. The lower face of the isthmus is very convex posteriorly , in front, the 

 tubercula bigeuiina are united to each other by a comparatively large transverse cord, 

 formed by the optic nerves intercrossing in the median line. The superior face is 

 depressed above the cerebellum, so as to constitute a fourth ventricle, also shaped like 

 the point of a pen : in front of this ventricle are the tubercula bigemina. These are 

 two voluminous tubercules separated from each other above, where they embrace the 

 cerebellum, and salient on the sides of -the lower face. They are hollow internally, and 

 communicate with the aqueduct of Sylvius , the thalami optici are little developed. 



The cerebellum is almost reduced to the median lobe, the lateral lobes, situated 

 behind and below it, being very small and conical. By its anterior extremity, this 

 cerebral ganglion passes between the corpora bigemini, and touches the cerebral hemi- 

 spheres. The cerebellum is annulated transversely to its surface, and between the 

 principal furrows are secondary ones, as in mammals. The white substance forms, in its 

 interior, an arborisation in relation, by the number of its branches, with the simplicity 

 observed on the surface of the organ. In the centre of the cerebellum of birds is a 

 small cavity communicating with the fourth ventricles. 



The cerebrum, divided into two hemispheres by a shallow sulcus, has the shape of 

 the heart on a playing card, more particularly when viewed on its lower face. The 

 convolutions are absent on the upper and lateral faces of the organ, and on the inferior 

 is a vestige of the fissure of Sylvius, which is directed obliquely forward and outward. 

 The olfactory lobes are little developed, and are placed together in the median line. 



The two ventricles are confounded, there being no corpus callosum nor septum 

 lucidum. There is no reflected portion in the ventricle ; consequently the hippocampi 

 and mastoid lobules are absent ; the corpora striata are, on the contrary, large and 

 occupy nearly the whole floor of the ventricles. 



CRANIAL NERVES. These are twelve pairs, as in mammals; and their origin is 

 analogous, if not identical; the only trifling differences pertaining to the pons Varolii 

 and the convexity of the lower face of the isthmus. 



Olfactory nerve. We have mentioned above how this is formed at the anterior 

 portion of the cerebral hemispheres. 



Optic nerve. It appears to be detached from the tubercula bigemini, and, after a 

 very short course, to intercross with that of the opposite side. In certain birds, especially 

 in the diurnal rapacious kinds, the optic nerves are constituted by fasciculi of undulating 

 nerve-tubes. 



