THE NERVOUS SYSTEM 521 



Isthmus 

 PRACTICAL STUDY 



9. Transverse Section through the Isthmus at the Exit of Nerve IV ( Trochlearis) 



(Figs. 332 and 353) 



In this there are to be distinguished three parts, the thin roof (superior medul- 

 lary velum), the tegmentum and, ventral to the latter, the pons. The tegmentum 

 consists essentially of the reticular formation, efferent cerebellar and midbrain 

 connections, and externally the afferent pallial connections. The pons contains 

 the efferent pallial paths to the cerebellum and to parts of the nervous system 

 caudal to it. The cavity is the iter or aquaductus Sylvii. Next to this is the 

 central gray of the brain wall. 



Efferent Peripheral Neurones. — The root fibres of the IV are seen in the roof. 

 They originate from nuclei lying further forward in the ventral part of the central 

 gray. The fibres pass from the nuclei dorsally and caudally in the outer part of 

 the central gray and finally decussate in the roof and emerge. It is only the 

 latter part of this course which is seen in this level. 



Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- 

 cephalic root of the \' lies in the lateral part of the central gray. Mingled with 

 its fibres may be seen the rounded cells, the axones of which form these fibres. 



The lateral lemniscus occupies part of the lateral swelling on the surface of the 

 tegmentum, forming the major part of the external structure known as the trigo- 

 num lemnisci. Groups of cells among its fibres constitute the dorsal nucleus of 

 the lateral lemniscus. The medial lemniscus is now still more flattened. The 

 spino-thalamic tract is in about the same position, between the two lemnisci. 

 Thus at this level, the principal afferent suprasegmental paths form an L-shaped 

 mass, enveloping the rest of the tegmentum and representing general bodily 

 sensation and hearing. There are also cranial nerve ascending paths lying 

 probably within the reticular formation and fillet (secondary vago-glossopharyn- 

 geal and trigeminal tracts, representing visceral, taste, and general head sen- 

 sation). These cannot be distinguished in the section. 



The ventral spino-cerebellar tract is on the surface, and now comes to lie 

 external to the superior cerebellar peduncle. At about this point it turns 

 caudally, and passes back into the cerebellum, accompanying the superior 

 peduncle. 



Otl er Afferent Cerebellar Connections. — The central tegmental tract oc- 

 cupies the same position. (For the pallio-cerebellar connection see ''Efi'erent 

 Suprasegmental Neurones" below.) 



Intersegmental Neurones. — The reticular formation is diminished in extent. 

 One of its nuclei, the nucleus centralis superior, lies near the raphe. The rubro- 

 spinal tract has moved somewhat mesially. It is dorsal to the medial lemniscus. 

 The medial longitudinal fasciculus is in the same position, and is a well-marked 

 bundle lying at the boundary between the ventral part of the central gray and the 

 reticular formation. 



Efferent Suprasegmental Neurones. — The pyramids are now broken up into 

 bundles which may show a tendency to gather in the ventral part of the pons 



