THE ROLE OF THE DORSAL TRACTS OF THE SPINAL CORD AND OF THE MESENCEPHALIC AND THALAMIC LEMNISCAL SYSTEM IN THE MILK-EJECTION REFLEX DURING MILKING IN THE EWE
SUMMARY The milk-ejection reflex after different lesions of the sensory system was studied by mechanical milking of the ewe. Only animals with lesions which did not produce locomotor ataxia were investigated. Section of the dorsal tract of the spinal cord at different thoracic levels blocked the milk-ejection reflex. This was not due to postoperative shock, because the same type of lesion made at the sacral level was without effect. Moreover, unilateral section of this tract blocked the reflex when milking was limited to the ipsilateral mammary gland. At the cervical level, the interruption of the dorsal tract was ineffective, which can probably be explained by the existence, in sheep, of the spino-cervico-thalamic tract. The latter becomes ventral at the cervical level. Lesions of the lemniscal system in the mesencephalon (bilateral coagulation of the medial lemniscus) and in the thalamus (bilateral destruction of the ventral posterolateral nucleus) do not inhibit the milk-ejection reflex. The role of the spinal lemniscal system is discussed in relation to the work of others and to the electrophysiological results obtained by one of us. It is suggested that two sensory systems (lemniscal and extralemniscal) have to interact to induce the release of oxytocin evoked by the stimuli of mammary origin during milking.