590 On a Cast of the Brain- cavity o/'Iguanodon. 



small ; it runs upward and outward, reaching the exterior 

 below and a little behind the fenestra ovalis {f.o.). About 

 1 cm. above and behind the root of the ninth is the large 

 vagus root (X.), which passes out of the skull by a passage 

 which, shortly after leaving the cranial cavity, forks, one 

 branch running backwards and outwards in the same direction 

 as the common base, the other directed forward and outward 

 at right angles to the first. The latter opens behind and 

 below the, fenestra ovalis at the bottom of the same depression 

 in the skull-wall ; the former just behind the prominent 

 oblique ridge forming tlie hinder boundary of the tympanic 

 depression. This posterior branch probably transmitted the 

 vagus (X.), and a considerable enlargement of its calibre just 

 external to the fork probably indicates the position of the 

 ganglion. As to what passed through the anterior branch there 

 is some doubt, but it may be suggested that it transmitted 

 a branch of the jugular vein (woodcut, ^'?<^.). About 15 mm. 

 behind and at the same level as the roots of the ninth and 

 tenth respectively, are two nerve-roots, of which the upper is 

 much the larger. These pass into the cranial wall by two 

 foramina, about 7 mm. apart, leading into passages which run 

 outward and backward, converging so that they have opened 

 close together in a common de})ression (woodcut, XII.). These 

 nerve-roots, I believe, both belong to the hypoglossal (XII.), 

 but perhaps the spinal accessory may also iiave passed out of 

 the skull by one of the foramina of which these prominences 

 are the casts. 



It will be seen that the determination of the regions of this 

 brain iiere adopted differs somewhat from that given by 

 Marsh in some of his figures of this organ in the American 

 Dinosaurs, the chief difi'erence being that while he regards the 

 middle prominence as representing in most cases a cast of the 

 optic lobes, I think that these probably did not project far 

 enough to leave any traces in a cast of the brain-cavity, the 

 enlarged middle ciiambcr of which was mainly occupied by 

 the cerebellum. In the crocodile certainly a cast of the 

 brain-cavity gives no idea of the form of the Ojjtic lobes. 

 Marsh's figure* of the brain of Ceratosaurus nasicoruis, in 

 which tlie optic lobes are marked as lateral structures not 

 appearing on the dorsal surface of the brain, probably most 

 nearly represents the actual condition of things. In the figure 

 of the brain of Claosaurus annectcns on the same plate the 

 nerve-roots marked agree in the main with the interpretation 



* Mar&b, "The Dinosaurs of North America" (Sixteenth Annual 

 l\eport of the U.S. Ueologieal Survey 1890), pi. Ixxvii. fig. 2. 



