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In an earlier publication (Schnitzlein, Rowe and Hoffman, '58) counts were reported of the myelinated nerve fibers in the mid-cervical vagus, recurrent laryngeal nerve and esophageal plexus of autopsy specimens of man. Although these numbers probably include most of the motor component of the vagus nerve, the number of non-myelinated nerve fibers, many of which arise in the nodose ganglion (Foley, '37), could not be determined from these preparations.Segments of nerves adjacent to those previously reported in the myelinated fiber study have been impregnated with pyridine silver to determine the numbers and percentage of non-myelinated nerve fibers in these preparations. Such estimations have been reported on the cat (Jones, '32; Du-Bois and Foley, '36, '37; Foley and Du-Bois, '33, '34, '37; Hoffman and Kuntz, '57) but the available literature regarding the vagus nerve of man does not include comparable data.It is the purpose of this study to compare the total number of axons in the vagus nerve with the counts of myelinated nerve fibers previously reported and to observe, in so far as possible, the distribution of the non-myelinated nerve fibers. MATERIALS AND METHODSSegments of mid-cervical vagi, recurrent laryngeal nerves after all the grossly observable thoracic branches have been given off, and the esophageal vagi at the diaphragm were removed at 17 autopsies. The nerves reported in this study are adjacent segments just proximal to those reported in an earlier publication (Schnitzlein, Rowe and Hoffman, '58). The same limitations which applied to the removal of the nerve segments utilized in the mye-h a t e d study applies to those reported in this investigation.The counts reported here were made following a pyridine-silver impregnation technique in the course of which they were gold toned and counterstained with Feulgen, fast green and orange G (Foley, '38). An ocular-grid estimation technique ( Schnitzlein and Foley, '57) was used to enumerate the fibers. The microscopes employed were equipped with oil-immersion objectives (120 X, N. A. 1.3) and 15 X oculars in order to obtain adequate resolution and magnification. Nearly all reported counts represent an average of at least two independent estimations. It should be noted that some sympathetic fascicles may possibly course with the vagus nerve at all levels. These fascicles are usually readily recognizable, especially at mid-cervical levels in the pyridine-silver preparations, and have been eliminated as far as possible from the counts.Comparable levels of the vagus nerve were obtained from Rhesus monkeys and cats. These were utilized for a electron microscopic study, particularly of the small non-myelinated "Remak bundles" (Gasser, '50). OBSERVATIONS Mid-cervical vagusThe total number of nerve fibers in the mid-cervical vagus nerve of man (table 1 ) varies from 45,110 to 153,123 fibers (right av. I= 105,375 3-6281; left av. = 87,379 -C 3948). In most instances the highest counts were recorded in the right vagus nerve but great variation exists in different i...

This investigation was undertaken in an attempt to present a description of the teleostean diencephalon compatible, in so far as possible, with the terminology more commonly employed, particularly for anurian, reptilian, and avian forms, and to illustrate some of the differences in the thalami of different bony fish. A number of descriptions of the teleostean diencephalon are available, but the names applied to the various nuclear groups and fiber bundles lack agreement from author to author and with that used for other vertebrates. The question arises as to whether such homologies are even possible. The earlier literature has been reviewed by Ariens Kappers C'06, '20) and by Ariens Kappers, Huber, and Crosby ('36). Only such mention of it will be made in the following account as is necessary for carrying out the purpose of the paper.The dorsal thalamus and the related nuclei of the fish vary considerably from order to order or even within an order (Franz, '12). Some of these variations may be more apparent than real. They may be due to shifts in position of analogous parts of the brain within the skull or referable to the relative differences in utilization of diverse sensory modalities and mechanisms of reaction. The gross appearance of the fish brain is not necessarily indicative of the degree of thalamic differentiation.The diencephalon of the teleosts has been described as convex with the convexity directed caudodorsally, so that the rostral part of the tegmentum of the mesencephalon and the optic tectum lie dorsal to the thalamus (Burr, '28). The exaggerated hypothalamus extends ventrocaudalward to the thalamus. The extensive postoptic (or suprachiasmatic) commissures lie ventral to the rostral part of the diencephalon. This slanting position of the diencephalon with respect to the longitudinal axis of the brain, the frequent absence of prominent ventricular sulci, and the thalamic extensions ventrolaterally and caudally from their ventricular relations contribute to the difficulties in interpreting the differences in the microscopic material of the teleostean thalamus. The presence, or size, or shape, of similar nuclei, particularly the nucleus rotundus, nucleus glomerulosus, and nucleus geniculatus lateralis, have added to the apparent disagreements.In attempting to homologize the terminology of the nuclei, three criteria have been used in all cases: (a) the location and the surrounding relations, including the sulci, the fiber tracts, and the nuclei; (b) the cell size and type and intrinsic histology of each nucleus including the arrangement of telodendrons; and (c) particularly the fiber interrelations with other nuclei.This preliminary paper will be limited to a consideration of the habenulae and the dorsal thalami of 4 fish: (1) a cypriniform ( suborder: Siluroidei), the tropical suckermouth catfish, Hypostomus plecostomus; ( 2 ) a clupeiform, the brook trout, Salvelinus f o n t i n a h ; ( 3 ) another cypriniform (suborder: Cyprinoidei), the goldfish, Carassius auratus; and ( 4 ) a perciform, the d...

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