Where a number of branches are given off together, the tracheal tube may be dilated. Fine branches, such as accompany nerves, are often sinuous. In the very finest branches the tube loses its thread, the chitinogenous cells become irregular, and the intima is lost in the nucleated protoplasmic mass which replaces the regular epithelium of the wider tubes.[150] [150] It has been supposed that these irregular cells of the tracheal endings pass into those of the fat-body, but the latter can always be distinguished by their larger and more spherical nuclei. _The Spiracles._ The spiracles of the Cockroach are by no means of complicated structure, but their small size, and the differences between one spiracle and another, are difficulties which cost some pains to overcome. The first thoracic spiracle (fig. 85) is the largest in the body. It lies in front of the mesothorax, between the bases of the first and second legs. It is placed obliquely, the slit being inclined downwards and backwards, and is closed externally by a large, slightly two-lobed valve, attached by its lower border. The aperture immediately within the valve divides into two nearly equal cavities, each of which leads to a separate tracheal trunk; and between these cavities is a septum, thickened on its free edge, against which the margin of the valve appears to close. A special occlusor muscle arises from the integument below the spiracle, and is inserted into a chitinous process which projects inwardly from the centre of the valve. A second muscle, whose connections and mode of action we have not been able to make out satisfactorily, lies beneath the first, and is inserted into the thickened edge of the septum. The second thoracic spiracle (fig. 86) lies in front of the metathorax, between the bases of the second and third legs. It is much smaller and simpler than the first. Its valve is nearly semi-circular, and the free border is strengthened on its deep surface by a chitinous rim, which terminates beyond the end of the hinge of the valve in a process which gives insertion to the occlusor muscle. [Illustration: Fig. 85.--First Thoracic Spiracle (left side), seen from the outside. × 70. _V_, valve; _I_, setose lining of valve (mouth of tracheal tube) × 230. The occlusor muscle is shown. The arrow indicates the direction of air entering the spiracle. In the natural position this spiracle is set obliquely, the slit being inclined downwards and backwards. (_P. americana._)]