Some misconception has prevailed, too, as to the function of the pericardial chamber. This space frequently contains a large quantity of fat-body—pericardial tissue—together with tracheae, and this has given rise to the idea that it might be lung-like in function; but, as Miall and Denny[58] have pointed out, this is erroneous; the tissues in Insects have their own ample supplies of air. It has also been supposed that the alary muscles cause the contraction of the heart, but this is not directly the case, for they are not attached to it, and it pulsates after they have been severed. It has been suggested that the contractions of this vessel are regulated by small ganglia placed on, or in, its substance. However this may be, these contractions vary enormously according to the condition of the Insect; they may be as many, it is said, as 100 or more in a minute, or they may be very slow and feeble, if not altogether absent, without the death of the Insect ensuing. The expulsion of the blood from the front of the dorsal vessel seems to be due to the rhythm of the contraction of the vessel as well as to its mechanical structure. Bataillon says,[59] confirming an observation of Réaumur, that at the period when the silkworm is about to change to the chrysalis condition, the circulation undergoes periodical changes, the fluid moving during some intervals of about ten minutes' duration in a reversed direction, while at other times the blood is expelled in front and backwards simultaneously, owing apparently to a rhythmical change in the mode of contraction of the dorsal vessel. {136}As the dorsal vessel consists of a number of distinct chambers, it has been suggested that there is normally one of these for each segment of the body; and it appears that the total number is sometimes thirteen, which is frequently that of the segments of the body without the head. The number of chambers differs, however, greatly, as we have previously stated, and cannot be considered to support the idea of an original segmental arrangement of the chambers. The dorsal vessel, though in the adult a single organ, arises in the embryo from two lateral, widely separated parts which only in a subsequent stage of the embryonic development coalesce in the median line. FAT-BODY.