Recently, the transverse areas of axons have been used as an index for pathologcal study based on the supposition that the transverse area of the axon is normally maintained constant. It is generally accepted that nerve conduction velocity is directly proportional to the fiber diameter , under the condition mentioned in the proceeding sentence. To verify this supposition, we measured the transverse areas of the facial nerve axons at three positions:proximal, middle and distal in three subjects. We found that the average crosssection area of the axons decreased moderately in proportion to the distance from the soma of each nerve. The diminution ratio averaged about 2 .6% per millimeter.

Summary: Comparative morphological and morphometric studies of the motor trigeminal nucleus (MTN) were carried out on the brains of an alligator, a human being and a rat for the first time. Differences in neurons regarding sizes, areas, numbers, column volumes, neuropil indices and circularity ratios were found. The results showed that the positions and shapes of the MTN were different, and the shapes of motor neurons were different, too. The average area of motor trigeminal neurons was the largest in the alligator, compared with the area in the human being, and the rat; the number of neurons and the column volume of MTN were the largest in the human being. As for the neuropil indices, the value was in descending order: alligator, human being and rat; whereas for the circularity ratio of neuronal cell bodies, the order was: rat, alligator, and human being. We concluded that the size of motor neurons in the MTN may be related to the power of masticatory muscles, and there may be a connection between jaw sizes and shapes and sizes of the motor neurons in the MTN.The motor trigeminal nucleus (MTN), which supplies motor fibres to the masticatory muscles responsible for controlling the closing and opening movement of the jaw, is an oval column of somatotopically organised cells (Palkovits & Brownstein, 1988), and consists of large motor neurons in the human being Mogoseanu et al., 1993;Palkovits & Brownstein, 1988).There have been reports about the human brainstem nuclei studied by morphometric method (Konigmark & Murphy, 1972;Moatamed, 1966;Monagle, 1974;Tomasch & Etemadi, 1962) and a few comparative studies concerning the motor neuron organization of the MTN in rats , as well as a morphometric study of the MTN in developing human brain (Hamano et al., 1988).In this study, we checked whether there were any morphological and morphometrical differences of the MTN among various animals including humans in relation to different jaw sizes. We chose the alligator, the human being and the rat as large, medium and small jaw samples. The position of the MTN innervating masticatory muscles is considered to be practically the same in various animals such as rats (Card et al., 1986;Limwongse & DeSantis, 1977;Mogoseanu et al., 1993;Weijs et al., 1993), cats (Mizuno et al., 1975), guinea pigs (Hecht et al., 1993;Uemura et al., 1982), rabbits (Matsuda et al., 1978) and macaque monkeys (Mizuno et al., 1981), although there might be slight differences in the MTN depending on the animal considered. No data, however, have been included in the literature concerning the morphometric analysis of neurons in the MTN from a comparative point of view. With the help of an image-analyzer and a computer, it is possible to get detailed data about nuclei of the brain stem. In this study the areas of neuronal cells of the MTN in the alligator, the human being and the rat were carefully measured. The relationship between jaw size and neuronal size of the MTN is discussed. Material and MethodsThe brains used in this study were as follows: a human b...