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Show 212 DR. R. W. SHUFELDT ON [Apr. I, The cerebellum is cup-shaped, smooth, and comparatively of large size ; its anterior concavity entirely covers the hinder portion of the optic lobes. Behind it, the dorsal aspect of the medulla oblongata is much scooped out, while its ventral flexure is but fairly well-marked. Upon carefully examining the roots of the cranial nerves, the foramen of Monro, the posterior commissure, the encephalic ventricles, and other minor structures of the brain-mass, I find nothing that might in any way be considered worthy of special record. I will say here, however, that I felt a strong desire to work out the cranial nerves ; they looked very tempting, but my material would hardly admit of it, as m y dissections of the eye, ear, tongue, and muscles of the head had already made extensive inroads upon this part of the bodies of m y several specimens, and in consequence the cranial nerves had to be frequently cut or broken up. Of I he Sacral and Brachial Plexuses.- Coming to the spinal nerves, the only ones to which we have paid any special attention in our subject are the branches that go to make up the brachial and sacral plexuses. These I observed quite closely. But upon studying the descriptions and examining the figures of these parts in a goodly number of species and genera of reptiles as given us by a great many anatomists, I have been forced to believe that these structures will never be anything more than uncertain ones in so far as they afford any reliable characters for classificatory purposes. Mivart speaks to the point in reference to this matter when he says, " As to the particular spinal nerves which go to form these plexuses respectively, and as to the mode of their interlacement and mode of giving origin to the limb-nerves, there is not only diversity between different genera of the same order and species of the same genus, but also between different individuals of the same genus, and even between the two sides of the same individual reptile" \ Regarding the brachial plexus in an adult specimen of Heloderma before me of the right side, I find that the fifth nerve that emerges from the spinal column, in addition to its sending off its smaller branches for muscular supply in its vicinity, also sends a long delicate branch which merges with the sixth spinal nerve, and so it constitutes the anterior part of the brachial plexus. The sixth, seventh, and eighth spinal nerves are very considerably larger than any of those that precede them or that immediately follow them, and they may be considered as constituting the main portion of the plexus. As they come out of the intervertebral foramina of the spine, the first two mentioned nerves pass over the posterior end of the rectus anticus major muscle, while the eighth spinalis still more extensively covered by the most anterior fasciculus of the retrahentes costarum series. Now the sixth spinal nerve as it approaches the shoulder-joint gives off four principal branches which supply various muscles of this region, and a little further on at its termination this is the fate of the main trunk itself. It, however, also sends off a short and rather thick branch that joins and merges with the trunk of the seventh nerve, before the latter anastomoses with the eighth. Following out- 1 Encyclopaedia Britannica, 9th edition, article '-Beptiles," vol. xx. p. 400. |