Besides the cerebellum, other organs that preside over reflex activity call for mention. Such organs, in the first place, are the thalamus and the corpora quadrigemina.
The centripetal paths of the thalamus are : the ascending tract of the medial fillet, the fibres of the optic tract ending within the pulvinar, the fibres from the olfactory centres and the fibres from the cerebellum by way of the superior peduncle. The thalamofugal paths lie within the tractus thalamo-spinalis, the tractus rubro-spinalis and the central tegmental tract. By means of the connections between the thalamus and the cerebral cortex, impulses coming from the periphery are carried to the cortex and, in reversed direction, activities occurring within the cerebrum are transferred to lower lying centres.
The centripetal path of the superior colliculus lies within the tractus opticus and partly within the lateral fillet ; that of the inferior colliculus within the lateral fillet. A centripetal path of the corpora quadrigemina is afforded also by the ascending tractus spino-tectalis, associated with the tractus spino-thalamicus. Fibres pass from the quadri geminal region to the cerebellum and an important descending path forms the tractus tecto-spinalis, the path from the quadrigeminal bodies to the spinal cord. Since fibres from the optic and acoustic nerves end within the quadrigeminal region and the path effects the transference of impulses of these nerves to the spinal cord, the tecto-spinal tract is also known as the reflex path.
The foregoing by no means completes the enumeration of the reflex paths, since throughout the brain-stem course numerous additional tracts, which serve to unite func tionally related centres. In this connection, it is only necessary to recall the complex mechanism of the medulla oblongata, in which different nuclei are brought into the most varied relations, whereby numerous simple, as well as the most complex, reflex proc esses are effected. While it is impracticable here to consider all such reflex paths, in order to obtain some notion of such complicated mechanisms, we may represent, by means of a simple diagram, the centres and tracts chiefly concerned in respiration.
Respiration is maintained by the stimulus carried to the respiratory centre through the circulation. In addition, the reflexes transmitted by the vagus also come into con sideration. In Fig. i8o, the respiratory centre is represented by the nucleus respiratorius within the formatio reticularis. This nucleus stands in close relation with the sensory end-nucleus of the vagus, since impulses are conveyed to it by the collaterals given off from the central vagus-tract. Moreover, as indicated in the figure, the nucleus respira torius is also under the influence of the higher lying respiratory centres. By means of the paths passing from the respiratory nucleus, as well as by the farther connecting neurones, the impulse is transferred to the motor nuclei of certain cerebral nerves and the gray substance of the spinal cord and, thence, is carried by the motor fibres to the muscles concerned in respiration. Thus, the impulse is carried by the phrenic nerve to the diaphragm; by the thoracic nerves to the intercostales and levatores costarum; by the cervical plexus to the scalene, sterno-hyoid and sterno-thyroid muscles (depression of the larynx); by the brachial plexus to the rhomboidei and pectoralis minor; by the accessorius to the sterno-cleido-mastoid and trapezius; by the vagus to the crico-arytaenoideus posticus and thyero-arytaenoideus (widening of the vocal cleft) and the levatores veli palatini et uvulae (elevation of the soft palate and the uvula); the facial nerve to the facial muscles (widening of the nasal apertures and the oral cavity). The paths passing from the nucleus respiratorius course in the medulla oblongata within the formatio reticularis, and, as shown in the accompanying diagram, numerous motor nuclei are brought into common activity by means of these association tracts (Fig. 220).