The Preoptic Region of the Brain and its Relation to Ovulation 103 



All animals were anesthetized with pentobarbital, 35 mg/kg body wt., 

 injected intraperitoneally. In proestrous rats the injection was given at 1.50 to 

 2.00 p.m. In the diestrous rats, the time of injection was often advanced 

 to 1.30. Sometimes as many as four rats were operated upon during the 

 following two hours and, thus, stimulation of the brain was brought about 

 at necessarily varying intervals after injection of the barbiturate. There 

 were no indications of variable results because of this. 



Fig. 1. Diagrammatic representation of the rat hypothalamus in sagittal section, from a 

 camera lucida tracing of a hemisected head. A concentric electrode is drawn to scale at 

 the upper left. Scale markings are in millimeters. The cranial floor and pituitary gland are 

 shown in broken lines, as are the midline projections of several nuclei. Key: AC — anterior 

 commissure, ARC — nucleus arcuatus, FX — fornix, MI — massa intermedia, MMB — mam- 

 millary body, OCh — optic chiasma, ON — optic nerve, PIT — pituitary, PO — preoptic nucleus, 

 PV — paraventricular nucleus, SC — suprachiasmatic nucleus, VM — ventromedial nucleus. 



While the animal was being placed in the stereotaxic apparatus, brief 

 supplementary anesthesia with ether was administered. This was sufficient 

 to inhibit reaction to exposure of the skull and drilling. 



Electrodes were stainless steel 36 gauge wires, Teflon insulated, in 0.40 mm 

 stainless steel tubing insulated with FormVar, Each element was bared 

 at the tip (cf. Fig. 1), and the wire extended slightly beyond the shield 

 (< 0.5 mm). Dual assemblies were prepared by soldering two of the con- 

 centric units in parallel to opposite sides of a shaft of steel tubing whose 

 diameter was appropriate to give the desired spacing of 1.2 to 1.5 mm. 



The stimulating current was taken from a Grass S-4C stimulator and 

 isolation unit with a 200 k^ resistor in each lead and with the electrode core 

 anodic. For monitoring current an oscilloscope was connected across a low 

 value resistor in series with one of the leads, and the system was calibrated 



