810 THE NERVOUS SYM'KM 



root-fibres of the spinal nerves, and find their nucleus of termination in a dorso-lateral 

 column of grey substance, which consists of the upward continuation of the gelatinous 

 substance of Rolando of the spinal cord. Opposite the entrance of the nerve is a con- 

 siderably thickened portion of this column of grey substance, known as the sensory 

 nucleus of the trigeminus, and the remainder below is called the nucleus of the spinal 

 tract (fig. 599) . After bifurcat ion the branches of the entering fibres of the trigeminus 

 terminate about the cells of these nuclei. The descending branches are much longer 

 than the ascending, and in passing downwards form the spinal tract of the trigemi- 

 nus, well marked in all transverse sections of the medulla oblongata (figs. 592, 593, 595, 

 600). The spinal tract decreases rapidly in descending the medulla, owing to the rapid 

 termination of its fibres in the nucleus of the tract. It has been traced as far down as 

 the second cervical segment of the spinal cord. The ascending branches being short, 

 most of them terminate in the 'sensory nucleus,' and, therefore, the upward exten- 

 sion of the nucleus of termination of the fifth nerve is both very short and scant. 



The nucleus of origin of the motor root of the trigeminus is also in two parts. 

 The principal nucleus lies on the dorso-medial side of the ' sensory nucleus/ a nucleus 

 of termination. It gives rise to the greater part of the motor root, and its fibres 

 are distributed to the muscles of mastication. Above the principal nucleus and along 

 the line of the locus cceruleus extends the nucleus of the mesencephalic (descending) 

 root. The cells of this latter nucleus are thinly scattered as high up as the posterior 

 commissure of the cerebrum, and the mesencephalic root arising from them gradually 

 increases (accumulates) as it passes through the mesencephalon to the superior level 

 of the pons, where it joins the fibres arising in the principal nucleus. 



The distribution of the fibres of the mesencephalic root is not clearly settled. 

 Collaterals from some of them are known to terminate about the cells of the principal 

 nucleus, and thus an impulse carried by them is not only given a wider distribution, 

 but is perhaps reinforced by the interpolation of another neurone. It is thought 

 that all of its fibres are not supplied to the muscles of mastication proper: some 

 may supply the tensor veli palatini and tensor tympani. (Kolliker) Both nuclei 

 receive cortical fibres from the pyramids. 



Axones from the nucleus of termination of the afferent portion of the trigeminus 

 are distributed (1) to the nuclei of its motor portion of the same and opposite sides 

 (short or simple reflex fibres); (2) to the nuclei of the motor cranial nerves; (3) 

 to the thalamus of the same and chiefly the opposite side, and thus, through inter- 

 polation of thalamic cells, their impulses reach the somsesthetic area of the cerebral 

 cortex. These fibres ascend in the reticular formation instead of coursing strictly 

 within the lemniscus. Fibres crossing the mid-line contribute to the internal 

 arcuates. 



The internal structure of the pons. The nuclei and roots of the trigeminus, 

 abducens, facialis, and acusticus are contained within the level of the pons, and their 

 position and course have been described above. The pons proper (the bridge) con- 

 sists of a mass of transversely running fibres continuous on either side into the 

 brachia pontis or middle cerebellar peduncles. In the animal series the relative 

 amount of these fibres varies with the size of the cerebellum, with which they are 

 connected. They are relatively more abundant in man than in other animals. 



In transverse sections the pons fibres are seen to course ventrally about the main 

 axis of the brain-stern, making it possible to divide the section into a basilar or 

 ventral part and a dorsal part (tegmentum). The fibres in their transverse and ven- 

 tral course around the medulla oblongata involve the pyramids. At the inferior 

 border of the pons the fibres little more than separate the pyramids as such from 

 the main axis of the brain-stem, but more anteriorly the pons fibres pass through the 

 pyramids, splitting them into the pyramidal fasciculi. These pyramidal or chief 

 longitudinal fibres of the pons are the continuation of the basal portion of the cere- 

 bral peduncles through the pons, to emerge as the pyramids proper at its inferior 

 border. They occupy an intermediate or central area among the pons fibres, leaving 

 the periphery of the pons uninvaded. The superficial pons fibres form the solid bundle 

 of its ventral and lateral periphery and the deep pons fibres form a similar bundle 

 dorsally enclosing the area of pyramidal fasciculi. 



In transverse sections through the inferior portion of the pons, the dorsal or 

 tegmental part consists of structures continuous with and analogous to the structures of 



