12 ON THE NERVOUS SYSTEM IT. 



VL Pituitary Body. (Hypophysis; Plate I. Figs. 1, 3, 7, 8, F.) This organ 

 is as universally present in the Vertebrate series as the preceding, and its function 

 is equally unknown. In Frogs it forms a very distinct portion of the encephalon, 

 is situated beneath the optic lobes at a short distance behind the optic nerves, and 

 is of an oval form with its greatest diameter in a transverse direction ; it has a short, 

 hollow membranous infundibulum, which is its only attachment to the base of the 

 brain, and is consequently easily separated by very slight force, when the orifice com- 

 municating with the ventricular cavity is easily seen. A body which has received 

 the name of tuber cinereum is found between it and the decussation of the optic 

 nerves, forms a slight projection from the under surface of the brain, and consti- 

 tutes the floor of the third ventricular cavity ; it is not, however, sufficiently well 

 defined to be regarded as a distinct organ. 



VII. Optic Lobes. (Plate I. Figs. 1, 2, 6, 7, 8, G.) In consequence of their form 

 and breadth, the optic lobes are the most conspicuous portions of the brain, and, 

 after the cerebral lobes, the largest. They consist of a pair of oval ganglia flat- 

 tened on their upper surface, their longest diameter being dijected outwards and 

 forwards, and on the median line, where the two come in contact and are united, they 

 are flattened, as it were, by mutual pressure. When seen from beneath (Fig. 1, 

 G), they make a prominent projection on either side of the pituitary body and the 

 tuber cinereum. After the pia mater has been removed, the optic tract can readily 

 be seen in its passage to the optic thalami and optic lobes. (I'late I. Fig. 1.)* 



VIII. Cerebellum. (Plate I. Figs. 2, 3, 6, 7, 8, H.) Immediately behind and 

 in part covered by the optic lobes, is the cerebellum ; which, when compared with 

 the same organ in the higher Vertebrates, presents a more striking contrast than 

 do either of the other cerebral masses with their respective homologues. In Ba- 

 trachians generally, the cerebellum does not seem to have passed beyond the mini- 

 mum of development for the class of Reptiles, an inferiority similar to that met with 

 in Petromyzon and Ammocoetes among Fishes. It consists of a flattened trans- 

 verse band of cerebral substance, continuous on either side with the medulla ob- 

 longata; its posterior border is slightly elevated (Figs. 7, 8, H), and forms the 



species, and is inserted between the " hollow lobes " (generally regarded as the optic lobes) and the 

 " anterior (or cerebral) lobes," by two small, either vascular or membranous medullary chords. (Lefons, 

 Tom. III. p. 135.) In the Salmonidtz, according to Agassiz, the pineal body consists of a plexus of very 

 slender vessels interwoven and anastomosing with each other, so as to form a club-shaped mass attached to 

 a slender pedicle. (Anat. des Salmones, p. 132.) According to Serres (Anat. Comp. du Cerveau, Tom. 

 II. p. 483), in Squalus squatina " it surpasses all the proportions which it is known to have in the other 

 classes." He also states, that " it is universally present in Fishes, but, in order that it may be detected, it 

 must be examined under water." In Tortoises it is much more distinct than in Frogs, is easily detected, 

 but having apparently the same vascular structure that it has in Batrachian Reptiles. Its presence in 

 Mammals and Birds is beyond a doubt, and in the former especially consists of a more solid mass, having 

 many points of structural resemblance to other parts of the encephalon. The calcareous concretions 

 which Soemmering, the Wenzels, Longet, and numerous other observers, have found to exist so 

 often, perhaps in nearly every instance, in the human body, are seldom or not at all found in the other 

 Vertebrates. 



* For pathological facts ahd physiological inferences with regard to the optic lobes, see the description 

 of the optic nerves. 



