and cardiac muscle, serving as a pathway for the spread of electrical excitation within a muscle cell Trachea: tubules forming the respiratory system of most insects and many arachnids Uriniferous tubules: any of the small tubules that are the excretory units of the vertebrate kidney Uveoscleral pathway: a tubule that drains excess aqueous humor Vasa efferentia: convoluted tubules that lead from the rete testis to the vas deferens and form the head of the epididymis Vesicular-tubular clusters between the golgi apparatus and endoplasmic reticulum which assist in transport. References Histology