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SCIENCE. 



NOTE ON THE "POSTERIOR BRAIN" OF 

 STEGOSAURUS. 



In a paper before the National Academy of Sciences in 

 November last, and more recently in an illustrated article 

 in the American 'Journal of Science, February, 1881, 

 Professor O. C. Mirsh has described certain remarkable 

 peculiarities of Stegosaurus ungulatus, one of the Dino- 

 saurian Reptiles of the American Jurassic formation. 



Judging from the figures, which are said to be reduced 

 to one-sixteenth of the natural size, the arm of this species 

 must have been about one meter in length, while the leg 

 was about twice as long. " The great disproportion in 

 size between the fore and hind limbs, as well as the 

 structure of the principal joints in each, show plainly that 

 Stegosaurus walked mainly as a biped. The massive 

 posterior limbs, and the huge tail doubtless formed a 

 tripod on which the animal rested at times, while the fore 

 limbs were used for prehension or defense. The heavy 

 dermal plates and powerful spines probably rendered the 

 latter an easy task." 



After recalling the statement which had been made by 

 him in a previous article, that, proportionately, " this 

 reptile had the smallest brain of any known land verte- 

 brate," Professor Marsh describes with some detail " a 

 very large chamber in the sacrum, formed by an enlarge- 

 ment of the spinal canal. This chamber was ovate in 

 form, and strongly resembled the brain case in the skull, 

 although very much larger, being at least ten times its 

 size. . . . A perceptible swelling in the spinal cord of 

 various recent animals has been observed in the pectoral 

 and pelvic regions, where the nerves are given off for the 

 anterior and posterior limbs ; and in extinct forms some 

 very noticeable cases are recorded, especially in Dino- 

 saurs. ... In some allied forms, Camptonotus for 

 example, where the disproponion between the fore and 

 hind limbs is nearly as marked, the sacral enlargement of 

 the spinal cord is not one-fourth as great as in Stego- 

 saurus. It is an interesting fact that in young individuals 

 of Stegosaurus the sacral cavity is proportionately larger 

 than in adults, which corresponds to a well-known law of 

 brain growth. The physiological effects of a posterior 

 nervous centre, so many times larger than the brain it- 

 self is a suggestive subject, which need not be here dis- 

 cussed. It is evident, however, that in an animal so 

 endowed, the posterior part was dominant." 



In the hope that Professor Marsh may continue his im- 

 portant observations and reflections upon this subject, 

 attention is called to the following points : 



1. It seems to be taken for granted that " the pos- 

 terior nervous centre" was as large as the sacral cavity. 

 This is hardly warranted, although it is certainly favored 

 by the size of the sacral foramina. The cranial part of 

 the elephant's skull is far larger than is required for the 

 lodgment of its brain, on account of the surface needed 

 for the attachment of the immense cervical muscles. 

 With many fishes, especially some skates and the Lo- 

 p/iius, the brain occupies but a small part of the capa- 

 cious cranial cavity. May it not be. then, that the sacral 

 cavity of Stegosaurus was enlarged, in part at least, in 

 correlation with a general enlargement of the whole 

 pelvis, in reference to the functions of the legs? 



2. Unless such examinations have been made already, 

 it would be well to ascertain the condition of the myelon 

 in the kangaroos, and that of the sacral cavity in 

 Diprotodon and Megatherium, all of which may be 

 compared with Stegosaurus in respect to the size of the 

 legs, or their employment in connection with a large 

 tail. 



It would be interesting to know the form and size 

 of the entire myelonal canal of Stegosaurus. The paper 

 leaves us in doubt as to whether the writer considers 

 the " posterior nervous centre " as the homologue of the 

 ordinary " lumbar enlargement" of the myelon in man 

 and other vertebrates. If not, may it be the not yet 



wholly abbreviated representative of what the late Prof. 

 Jeffries Wyman referred to in his paper " On Symmetry 

 and Homology in Limbs"? In some adult fishes the 

 spinal marrow ends in a ganglionic enlargement forming 

 a kind of caudal brain. We have found such a ganglion 

 quite conspicuous in the American Lophius." In either 

 case, it is probable that the remarkable condition of 

 things in Stegosaurus, as described by Prof. Marsh, 

 would have appeared to Prof. Wyman as an example of 

 the law of organic polarity in the form of " fore-and 

 hind symmetry," which has been advocated by him, by 

 Dr. Coues, and by the writer of the present notice. 



B. G. W. 



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ATMOSPHERIC DUST. 



At the period of the great debate on spontaneous genera- 

 tion between M. Pasteur and Pouchet, the latter was the 

 first to draw attention to the fact, that some of the minute 

 spherical granulations, discovered by the microscope in 

 dust deposited from the air in various regions of the globe, 

 were essentially composed of silica. That they had often 

 been mistaken for eggs of infusoria or- for micrococci was 

 very evident ; but when the dust was submitted to complete 

 calcination in a platinum crucible the same grains were 

 still visible, with the same forms and dimensions as be- 

 fore. 



I have more than once repeated this experiment of 

 Pouchet's, but I have also made the opposite one and 

 examined the action of heat upon micrococci, diatoms and 

 oscillaria;, which are supposed to contain large quantities 

 of silica. 



There is no doubt that the dust of the atmosphere reveals 

 to the microscope, besides the larger mineral fragnents 

 mostly of an angular shape, exceedingly minute circular or 

 spherical bodies, being often not more than 0.001 of a milli- 

 metre in diameter, and very similar in size and shape, which 

 resist the action of a white heat in contact with the air, 

 and that of strong hydrochloric acid. In some of my ob- 

 servations they were remarkably numerous. Both before 

 and after the action of heat they are more or less transpar- 

 ent. What can be the origin of these singular objects? 



The same experiments repeated with siliceous algae, such 

 as those belonging to the large family of the diatomacea, and 

 with the micrococci of impure waters or vegetable infusions, 

 showed me that they do not retain their forms after being 

 subjected to the above tieatment, and that in many in- 

 stances they can be totally destroyed by heat on the object 

 glass itself. On the other hand, the fossil diatoms resisted 

 the action of heat and acids and retained their forms. I 

 can draw only one conclusion from these observations : 

 namely, that the minute siliceous bodies found in the at- 

 mosphere are also fossil — they are micrococci of another 

 age— Dr. T. L. Phipson, F.C.S. 



Silver Bromide. — The action of light upon this sub- 

 stance varies according as the bromide is in the state of 

 emulsion in an indifferent medium, like collodian, or in an 

 organic substance readily oxidisable, like gelatine. Tem- 

 perature, moisture, and mechanical pressure do not appear 

 to have any influence. 



