300 



AVES. 



the human brain ; they have most resemblance, 

 however, to the latter bodies. 



With respect to the substance of which the 

 brain of birds is composed, we may observe 

 that the bodies analogous to the corpora striata 

 do not merit that name, as there are no alterna- 

 ting striae of grey and white matter. In this 

 respect the bird's brain resembles that of the 

 cold-blooded ovipara and of the human foetus. 

 The substance of the cerebellum does present 

 the admixture of the two substances, or arbor 

 vita (q), but in a less complicated degree than 

 in mammalia. 



The brain in birds is invested with the same 

 membranes as are described in Mammalia. 



Medulla spinalis. The spinal cord is con- 

 tinued from the foramen magnum to the canal 

 formed by the coccygeal vertebrae, where, how- 

 ever, it becomes extremely attenuated, and 

 corresponds in extent to the shortness of that 

 division of the vertebral column, terminating 

 in a mere filament which expends itself in 

 distributing a few pairs of nerves through the 

 coccygeal foramina. As in the Mammalia, it 

 appears externally to be composed of the white 

 or medullary matter, but contains a small pro- 

 portion of grey substance internally. It is of 

 a cylindrical figure, and as in the cold-blooded 

 ovipara, it is of great length in proportion to 

 the brain. An anterior and posterior fissure 

 may be distinguished, and also a narrow canal 

 which extends through its entire length. Two 

 enlargements occur in the course of the spinal 

 cord, one corresponding to the wings, the 

 other to the legs ; and from these swellings 

 the nerves of the brachial and sacral plexuses 

 come off respectively. As might be expected, 

 therefore, these enlargements present differ- 

 ences of relative size corresponding to the dif- 

 ferent relative development and powers of the 

 anterior and posterior extremities. In general 

 the posterior enlargement is greater than the 

 anterior; and this difference is very remarkable 

 in the Struthious birds in which the whole 

 business of progression falls upon the posterior 

 extremities. 



Besides the difference in size, the spinal 

 enlargements or ganglions, as they may be 

 termed, differ also in structure ; at the anterior, 

 alar, or thoracic enlargement (r, Jig. 134) the 

 spinal cord merely receives an accession of 

 grey and white medullary substance ; but at 

 the beginning of the sacral swelling (s,fig. 134) 

 the canal of the cord enlarges in a remark- 

 able manner, so that the lateral cords separate 

 from one another posteriorly or above, pre- 

 cisely as they do to form the fourth cerebral 

 ventricle : the cavity or spinal ventricle (s, 

 Jig. 134) thus formed, is filled with a serous 

 fluid inclosed in a pia mater. From the figure 

 of this cavity it has been termed the ' Sinus 

 rhomboidalis.' 



Of the Nerves. The cerebral nerves cor- 

 respond in number to those of the Mammalia. 

 The principal difference of form and structure 

 is presented in the olfactory or first pair 

 (I, Jig. 135.) These nerves are of a cylin- 

 drical figure and small extent, being continued 

 from the anterior extremity or apex of the 



hemispheres. Instead of separating into fila- 

 ments to pass out of the skull by a cribriform 

 lamella, each nerve is continued along an 

 osseous canal, accompanied by a venous trunk, 

 as far as the pituitary membrane of the supe- 

 rior spongy bone upon which its filaments are 

 distributed in a radiated manner. 



The optic nerves (2, figs. 135, 137,) are in 

 general of remarkable size ; they arise from the 

 whole of the outer surface of the optic lobes, 

 and form in front of the infundibulum, a perfect 

 union, or chiasma, (2*, Jig. 137,) in which, on 

 making a horizontal section, some transverse 

 striae may be perceived, apparently resulting 

 from the decussating fibrils of the nerves. 



The distribution of the third, (3, Jigs. 135, 

 137,) fourth, (4, figs. 135, 137,) and sixth 

 cerebral nerves, (Q, Jigs. 135, 137,) is almost 

 the same as in Mammalia. The course of the 

 fourth pair, immediately above the supra- 

 orbital branch of the fifth pair is shown at 

 4*, fig. 137, as far as its termination in the 

 superior oblique muscle to which it is, as in 

 other vertebrata, exclusively distributed. 



The fifth or trigeminal nerve (5, figs. 135, 

 137) has nearly the same distribution as in 

 Mammalia. 



The first or ophthalmic division (5*, fig. 137) 

 passes out of the cranium by a peculiar canal 

 situated externally to the optic foramen. It is of 

 large size, and describes in its passage through 

 the orbit a curve corresponding to the roof of 

 that cavity; it generally penetrates the substance 

 of the facial bones above the nasal fossae. It 

 divides into three branches ; the first or supe- 

 rior is the smallest and is lost upon the pitui- 

 tary membrane ; the second branch is the 

 largest of the three and the longest ; it is re- 

 ceived into an osseous canal, passes over the 

 nasal organs, and terminates at the extremity 

 of the beak in a great number of divisions ; 

 the third branch of the ophthalmic nerve is 

 entirely distributed to the skin which covers 

 the circumference of the external nostrils. 



The second division, or superior maxillary 

 nerve passes out of the same foramen as the in- 

 ferior one (at 5", Jig. 137,) immediately above 

 the tympanic bone or os quadratum ; it passes 

 forwards along the floor of the orbit, and in 

 this part of its course gives off two filaments, 

 of which one joins the ramifications of the 

 ophthalmic nerve, the other ascends, penetrates 

 the substance of the pterygoid muscles and 

 the maxillary bone, to be lost on the lateral 

 parts of the bill. In those birds, as the 

 Anatida and other Water- fowl, where the upper 

 mandible is notched on the edge, each denticu- 

 lation receives four or five nervous filaments, 

 and the nerve is proportionally of large size. 



The inferior maxillary nerve separates from 

 the superior, and proceeds obliquely down- 

 wards, dispensing branches to the pterygoid 

 and quadrangular muscles of the jaws; the 

 trunk proceeds outwards to the lower jaw 

 where it divides into two branches an internal 

 and an external. The internal, which is a con- 

 tinuation of the trunk, penetrates the maxillary 

 canal, and is continued to the anterior end of 

 that mandible. In the Anatida it gives off 



