92 PHYSIOLOGICAL SERIES. 



be seen the cut surface of the nervous bridge that unites 

 the two posterior cniru of the cerebellum, and behind this 

 the lohus facialis and lobus vagi. 0. C. 1308 0. 



D. 90. A .-imilar >|>ecimen in which the structure of the optic 

 lobes and valvula cerehclli is more clearly visible. 



In this specimen the left optic nerve crosses above the 

 right. 



D. 91. The brain of a Cod ( Gadus morrhud) dissected from above. 

 The posterior lobe of the cerebellum and the tectum 

 opticuin with the exception of the torus longitudinalis 

 have been removed, thus exposing the medulla and the 

 cavity of the mid-brain. Within the optic ventricle are 

 certain marked prominences : on either side a large semi- 

 lunar excrescence (torus semicircularis) due to the presence 

 at this spot within the latero- ventral walls of the mid-brain 

 of a nucleus in connection with the fillet : in the central 

 mid-line at the hinder end of the ventricle a subconical 

 projection (valvula cerebelli), the structure and relations 

 of which are shown in the previous specimens : and in the 

 dorsal mid-line a pointed tongue-like process that extends 

 from the posterior commissure to the hinder end of the 

 tectum opticuin. This structure, composed of the two 

 tori longitudinales, consists of nerve elements and a local 

 thickening of the ependyma that lines the brain-cavities. 

 Beneath its broad anterior end lies the narrow opening 

 by which tin- optic ventricle communicates with the 

 aqueduct of Sylvius. 



The dorsal surface of the medulla is occupied by a series 

 of swellings that almost entirely close the rhomboid fossa, 

 and are due mainly to the sensory nuclei of the cranial 

 ner\-- (1JM. 2'i). .fudging by Goronowitsch's account of 

 these structure- in /,<>t<i, their relation to the several cranial 

 nerves .-hoiild lie approximately as follows : The prominent 

 lateral crura of the cerebellum contain hoide-; cen-bellar 

 tracts, a pair of large nuclei from which arise the acoustic 

 and lateral-line m-rves. These, lobes apparently represent 

 the tubcrciilum acii.-ticutn of Klasmobranchs (Johnstone, 

 Zool. .Jalirk l'.0|,p. I.V.I) or the Sturgeon shifted forward 

 owing to the concentration of the medulla. Close behind 



