75i 



SCIENCE. 



[X. S. Vol. XXI. Xo. 541. 



and pole changer. It is in fact a universal 

 key. Xo mercury is used. 



The central binding posts are prolonged 

 upwards and each is slotted to receive a brass 

 bar which is pivotted in the slot by a hori- 

 zontal pin. The brass bars are held parallel 

 by two rubber rods which serve as handles. 

 When the bars are depressed to one side or 

 the other, they engage between plate's of spring 

 brass set into brass blocks each of which car- 

 ries a binding screw. Cross wires enter these 

 blocks, as shown in the figure. At one end 

 the cross wires are soldered into the blocks, 



thus making an electrical contact. The two 

 blocks at the other end are perforated by rub- 

 ber cores or ' bushings ' through which the 

 cross wires pass. The cross wires, therefore, 

 make no electrical contact with these blocks. 

 When a contact is desired, the screw borne on 

 the head of each cross wire is turned until its 

 face presses against the brass block outside the 

 bushing. In this position the key serves as a 

 pole changer, commutator, or ' Wippe.' 



A brass cross bar unites the central posts. 

 At one end this cross bar does not make elec- 

 trical contact with the post, but passes through 

 a rubber bushing clearly shown in the figure. 

 Contact is secured by turning a screw upon 

 the cross bar until the face of the screw presses 

 against the post outside the bushing. When 

 this contact is made the instrument may be 

 used as a short-circuiting key. 



W. T. Porter. 



Harvard Medical School, 

 some notes on the myodome of the fish 



CRANIUM. 



Myodome is a term given by Dr. Theo. Gill 

 to the tube at the base of the cranium of 



fishes for the reception of the rectus muscles 

 of the eye. 



It is formed by an inner longitudinal wing 

 springing from near the middle of the prootic, 

 or near the center of the radiation of the 

 structural fibers of the prootic, and meeting 

 its opposite fellow separates the myodome 

 from the brain cavity. 



It has been variously called eye muscle tube, 

 canal or vacuity, and Cope instead of consid- 

 ering the vacuity itself considered the walls 

 forming it, thus : ' basis cranii double,' mean- 

 ing a double floor to the cranium with a space 

 (the myodome) between ; ' basis cranii single,' 

 or myodome absent. With the former term 

 he often coupled the phrase, ' with a muscular 

 tube,' meaning the vacuity was extended back- 

 wards in a tube in contradiction to its ending 

 blindly. 



The following matter was suggested to me 

 by a few lines in an excellent paper recently 

 published by Dr. W. G. Ridewood (Proc. Zool. 

 Soc. London, 1904, Vol. II., p. 60) as follows: 



Xo value can be ascribed, so far as I can see, 

 to a feature upon which Cope has laid some stress 

 {Trans. Amer. Phil. Soc, X. S., XIV.. 1871), 

 namely the double or simple nature of the basis 

 cranii. This refers, so far as I understand his 

 writings, to the separation of the parasphenoid 

 from the prootic floor of the cranium by the eye 

 muscle vacuity. The character is one which is 

 very difficult of application; and it is a matter 

 of individual opinion whether such a form as 

 Clupea is to be regarded as having a simple or 

 double bases cranii, for here the parasphenoid is 

 produced backwards into a pair of large lateral 

 wings, the space between them freely open below. 



From the second sentence quoted, I should 

 judge that Dr. Ridewood considers the myo- 

 dome to be interposed between the floor of the 

 cranium and the parasphenoid, rather than 

 separated from the brain cavity. Or, in other 

 words, that he considers the roof of the myo- 

 dome to be the homologue of the cranial floor 

 of forms having no myodome. 



This does not seem to me to be the correct 

 conception. That the floor of the myodome 

 is the true cranial base and that the roof of it 

 is simply a septum of secondary development 

 would seem probable from the following evi- 

 dence: (1) The lower edges of the prootics 



