The pre-and postganglionic cardioacceleratory innervation is described in the pigeon. The peripheral coursc of the postganglionic cardiac nerves has been determined using microdissection and electrical stimulation. Using these techniques and retrograde degeneration methods, the distribution within the sympathetic ganglia of the cells of origin of these fibers has been localized to the three right caudal cervical ganglia (12, 13 and 14). It has also been shown o n the basis of electrical stimulation combined with selective ablation of the right sympathetic chain that cardioaccelerator preganglionic fibers probably arise from the most caudal cervical segment of the spinal cord (14), always arise from the upper two thoracic segments (15 and 1 6 ) , and occasionally arise from a mid-thoracic segment (17). The left sympathetic chain was shown to have an inconsistent influence o n heart rate. On the basis of retrograde degeneration, the cells of origin of sympathetic preganglionic fibers have been localized to a welldefined cell column dorsal to the central canal (column of Terni).This paper is the third in a series of studies designed to describe the cells of origin of vagal and sympathetic fibers controlling heart rate in the pigeon. While the preceding two papers have dealt with the dorsal motor nucleus and peripheral vagus nerve (Cohen, Schnall, Macdonald and Pitts, '70; Cohen and Schnall, ' 7 0 ) , the present report is concerned with the sympathetic innervation of the heart. Specifically, this study was directed toward describing ( a ) the peripheral course of the postganglionic cardioaccelerator nerves; ( b ) the distribution within the sympathetic ganglia of the cells of origin of these fibers; ( c ) the spinal segments contributing cardioacceleratory preganglionic fibers; and ( d ) the localization within the spinal cord of the cells of origin of sympathetic preganglionic fibers.
MATERIALS A N D METHODS
Dissection of sympathetic nerves and the paravertebral chainDissections were performed on 25 White Carneaux pigeons using a Zeiss Operation Microscope (1OX-16X). All birds were sacrificed by intraperitoneal administration J. COMP. NEUR, 140; 343-358.of an overdose of sodium pentobarbital. In five cases dissection was undertaken immediately upon sacrifice. However, the remaining 20 cases were perfused through the left ventricle with avian saline (0.75%) followed by 10% formalin, and dissections were performed after one to seven days of refrigeration.The surgical approach to the paravertebra1 chain and peripheral sympathetic nerves was as follows. A dorsal midline incision was made from segments 10-19 ' (middle cervical to lower thoracic) and the skin reflected laterally. All muscles inserting on the vertebral column were severed at the midline to allow the muscles and scapula to be reflected laterally and held in position with a retractor, This procedure exposed the intercostal muscles of segments 15-18 and the brachial plexus of
