Receptor Elements in the Thoracic Muscles of Homarus vulgaris and Palinurus vulgaris

Abstract

1. In the thoracic muscles of Homarus vulgaris and Palinurus vulgaris the presence of receptor elements of various kinds has been recorded. Muscle receptor organs belonging to the same category as described previously in the abdomen have been found in the two posterior (7th and 8th) thoracic segments. According to their situation one lateral and one median receptor may be distinguished on each side of each segment. Like those in the six abdominal segments they are linked with the system of the extensor muscles. The topography of these organs in relation to that of the thoracico-abdominal muscles is described. There are certain differences in their location between the two species. 2. Receptor organs of this category, with one exception, are composed of a special thin muscle and a nerve-cell ending in connective tissue intercalated in this muscle. The lateral receptor of the 7th thoracic segment in Homarus has no muscle of its own and terminates on connective tissue fibres accompanying an ordinary muscle. In the nerve supply of these receptors in the thorax the same elements as in the abdomen, i.e. the motor nerves and the two accessory nerves, can be distinguished, but their distribution in the thorax has certain special features. 3. It is assumed, in view of certain differences in the appearance of the nerve-cells and their processes, that the muscle receptor organs of this category are of two types in each of the segments. 4. Nerve-cells regarded as receptors of a different category have been found in some of the muscles inserting in the median surface of the epimeral plate. These cells, termed ‘N-cells’, are smaller elements than those of the first category and have not a special muscle of their own, but end with long processes between the fibres of the ordinary muscles. In each of the species investigated five such elements have been found. No evidence is as yet available as to whether they are present exclusively in the thoracic region or are more generally distributed. 5. It is suggested that the N-cells may represent more primitive forms of muscle receptors, and that the receptor organs of the extensor muscles in the thorax and abdomen are more highly evolved forms.