A study was made of the decline in the number of motor neurons and axons of the brachial spinal cord of the rat during postnatal development. The injection of horseradish peroxidase (HRP) into the biceps muscle showed that it was innervated by motor neurons located in the dorsolateral position of the lateral motor column in segments C5 and C6; HRP injections into the triceps muscle showed that it was innervated by motor neurons located in the ventrolateral position of the lateral motor column in segments C7 and C8. There was no change in the position of these motor neuron pools between birth and maturity. However, there was a decline in the number of neurons in each pool during the postnatal period; over 35% of the neurons present at birth had disappeared by maturity. This loss of neurons was uniform throughout the rostrocaudal extent of each pool. It was accompanied by a similar percentage loss in the number of axons in a ventral root at the branchial level (C8). Electrophysiological measurements showed that the disappearance of motor neurons was accompanied by a loss in the polyneuronal innervation of synaptic sites in the biceps muscle. The possibility that a decrease in the number of neurons contributes to the loss of polyneuronal innervation is discussed.

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The pharyngeal hypophysis and the pars distalis of the hypophysis cerebri were examined in each of 52 cadavers of a mean age of 75 years. Comparative studies were made of volume and histology. The pharyngeal hypophysis was found to be of appreciable volume and to represent up to 0.5% of the total adenohypophysial tissue in the cadaver. The pars distalis in the female was significantly larger than that in the male, but no significant sex difference in size was noted in the pharyngeal hypophysis. In the majority of subjects, the histological appearance of the pharyngeal hypophysis closely resembled that of the pars distalis. The findings of this investigation demonstrate that, on the basis of size and cytology, the pharyngeal hypophysis is of functional significance in the presence of an aged, normal pars distalis. The suggestion is made that the functional significance of the pharyngeal hypophysis may increase with age, particularly in the male.

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