THE FORM AND SUBDIVISIONS OF THE BRAIN 23 



represented by an accessorius branch of the vagus. The XII nerve is 

 represented by branches of the first and second spinal nerves. The 

 first spinal nerve in some specimens has a small ganglion ; the second 

 nerve always has a large dorsal root and ganglion. In this connection 

 a passage in the comprehensive work on the anatomy of Salamandra 

 by Francis ('34, p. 134) is worthy of mention: "After making due 

 allowance for the absence of a lateralis component in the adult 

 salamander, the correspondence between the cranial nerves of this 

 animal and those of Ambly stoma is very close indeed." 



The configuration and mutual relations of the gross structures just 

 surveyed can be seen only in sections, of which many, cut in various 

 planes, have been illustrated in the literature. Only a few selected 

 examples are included in the present work, with references in subse- 

 quent chapters to many others. For general orientation the following 

 figures may be consulted : a series of selected transverse sections from 

 the spinal cord to the olfactory bulb (figs. 87-100); a series of hori- 

 zontal sections through the middle part of the brain stem (figs. 

 25-36); a few sagittal sections (figs. 101-4). Figures 6-24 show the 

 chief fibrous connections of each well-defined region of the brain 

 stem. 



The diencephalon, mesencephalon, and isthmus have the form of 

 three irregular pyramids oppositely oriented (fig. 2A). The broad 

 base of the diencephalon extends from the anterior commissure to the 

 hypophysis, and the apex is at the epiphysis. The tectum forms the 

 base of the mesencephalic pyramid, and the apex is at the ventral tip 

 of the tuberculum posterius, which borders the ventral cerebral 

 flexure. The base of the pyramidal isthmus is formed by the massive 

 tegmentum isthmi of each side and the median interpeduncular 

 nucleus in the floor plate. It narrows dorsally into the anterior medul- 

 lary velum between the tectum and the cerebellum. 



The middle sectors of the brain stem — diencephalon, mesenceph- 

 alon, and isthmus — contain the primordial regulatory and in- 

 tegrating apparatus controlling the fundamental sensori-motor sys- 

 tems of adjustment. The most important peripheral connections are 

 with the eyes, and these in most vertebrates play the dominant role 

 in maintaining successful adjustment with environment. From this 

 topographical feature it naturally followed that, during the course of 

 phylogenetic differentiation of the brain, the chief centers of adjust- 

 ment of the other exteroceptive systems were elaborated in close 

 juxtaposition with the visual field in the midbrain and thalamus. 





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