This physiology of the passenger terminal has dwelt so far upon its brain and its nerve structure; the anatomy is hardly less interesting. Almost every great passenger terminal in America is built upon the head-house plan. In this scheme trains arrive and depart upon a series of parallel tracks terminating within some sort of train-shed. It is the ideal scheme from the standpoint of the passenger, for no stairs or bridges or subways are necessary to reach any track. The tracks are generally laid in pairs, and between each pair a broad platform is built, which is in reality a long-armed extension of a common distributing platform or concourse extending across the head of the tracks. Sometimes these extension platforms are laid on both sides of a single track for greater facility in handling baggage and for the quick unloading of heavy trains. But in case any number of trains are to be operated through the terminal, the head-house scheme becomes impracticable and an abomination to the operating department. It makes necessary all manner of backing and turning trains and a tremendous amount of energy and time is spent in so doing. So we find the head-house stations--the real terminals of America--for the most part along the seaboard or at the termination of really important railroad routes. They are an expensive luxury at any other point. At the outer end of the train-shed, its tracks begin to converge. They are in rough similarity to the sticks of an open fan and at the handle they are reduced to anywhere from two to eight main tracks, the connections with the through tracks that serve the station. The point of convergence is known to the towerman and all the other workers as the "throat" of the yard. It is by far the most important point of the terminal, and is the usual location of the control tower, with its authority over several hundred switches and signals. Upon the number of main tracks in this "throat" depends the capacity of the terminal, quite as much as the number of tracks in the train-shed or the size of any other of its facilities. If there are as many as eight tracks in this "throat"--an unusual number--the signals and switches will probably be arranged so that in the morning five tracks may be used for the rush of incoming business, and three tracks for outgoing business, while in the late afternoon conditions are exactly reversed, five tracks being used for hurrying the suburbanites homeward, three for the lesser business incoming to the terminal. With four tracks in the "throat"--a usual number--three may be used in the direction of the volume of greatest business. Each of these tracks is like a separate entrance to the terminal, and when five are open from the train-shed simultaneously, as in this first case, five outgoing trains may be started simultaneously from as many tracks. In this connection, a comparative table of the capacity of several of the largest American passenger terminals may not be without interest: