June 29, 1883.] 



SCIENCE. 



613 



VERTEBRATES. 



Development of the pulmonary epithelium. — 



The lungs of the human adult have been minutely 

 studied by Kolliker, whose memoir, which appeared 

 in 1881, still left the development of the lung to be 

 ■worked out. This gap has now been partially filled 

 by Nicolai Jalan de la Croix, who, however, has relied 

 on the chick and mammalian embryos for the earli- 

 est stages. In a human embryo of the third month 

 (6.5 cm. ) the bronchi are nearly straight tubes branch- 

 ing at acute angles; the alveoli have begun to form 

 at their ends, but are developed in the inner part of 

 the lung only later; the connective tissue is in process 

 of differentiation; the whole system of respiratory 

 cavities is lined by a continuous epithelium, which 

 ■ is thickest in the trachea, where it has several layers 

 of cells, and which gradually thins out, until, in the 

 alveoli, it consists only of two layers of cells, the 

 deeper cells being somewhat smaller, the upper ones 

 irregular in shape, and approaching the cylindrical 

 form. The alveoli are already grouped into lobules; 

 and it is these which Kolliker has described in his 

 embryology as the primitive alveoli. By the end of 

 the fourth month the bronchi branch off at much 

 greater angles; the epithelium in the terminal vesi- 

 cles is only 1.5 ji thick, and consists of a single row of 

 cylinder cells. In the fifth month the connective 

 tissue around the bronchi is quite advanced in devel- 

 opment; it is, between the lobules, largely fibrous; 

 between the alveoli, still rich in cells. The alveoli 

 themselves measure about 0.0.5 mm. in diameter; i 

 their epithelium, only 11 ^ in thickness. The blood- 

 vessels have attained an enormous development, but 

 are not yet close to the respiratory surfaces. Com- 

 parison of the different stages shows that the alveoli 

 gradually increase in number, and at the same time 

 diminish in size (author's resume, vide p. 109). 

 The conversion of the many-layered original epithe- 

 lium into the single layer of the alveoli, the author 

 asserts (apparently without definite reason) to be 

 effected by the passage of the deeper-lying cells into 

 the upper layer. By this process, as well as by the 

 mulliplication of the cells, is the rapid expansion of 

 the epithelium to be explained. For the history 

 during the fifth to ninth month, de la Croix col- 

 lates the previous literature. 



In the mature foetus (still-born) alveoli are still 

 forming along the alveolar canals. The epithelium 

 of the canals and all alveoli is still cylindrical, the 

 cells with oval nucleus being about twice as high as 

 broad. The alveoli do not yet extend down into the 

 meshes of the capillary net-work. In a child that 

 lived for seven days the flattening-out of the alveolar 

 epithelium had already made considerable progress 

 (Stieda found that this flattening took place much 

 earlier in sheep embryos). The very rapid develop- 

 ment of the pavement out of the cylinder epithelium, 

 the author says, must be necessarily produced by the 

 expansion of the lungs after birth. (There are two 

 objections to this view, — first, it is not shown that 

 the change accompanies an expansion; second, it fails 

 to account for the development of the flat cells during 

 foetal life, as in sheep. Rep.) — [Arch. mikr. anat., 

 xxii. 9.3.) c. s. M. [1176 



The nature of inhibition. — Professor T. Lauder 

 Brunton has lately offered a theory of inhibition 

 founded on its analogy to the interference which 

 occurs when waves of light or sound meet in oppo- 

 site phases. According to his hypothesis, there are, 

 in the cord and brain, successive layers of sensory 

 and motor cells, so arranged that each motor cell is 

 connected, not only with its corresponding sensory 



cell, through which the afferent impulse causing a 

 simple reflex first passes, but also with other sensory 

 cells higher or lower in the cord. When the afferent 

 nerve leading to a sensory cell is slightly stimulated, 

 a simple reflex occurs through the corresponding 

 motor cell. So when several afferent fibres are gen- 

 tly stimulated, as in tickling the sole of the foot, the 

 impulse from each sensory cell passes to a motor cell, 

 and calls forth a reflex contraction. If the afferent 

 fibre leading to any sensory cell is more strongly 

 stimulated, the impulse on reaching the sensory cell 

 will divide, part going directly to the motor cell, part 

 passing to a neighboring sensory cell and thence in- 

 directly to the motor cell. The consequence is, that 

 the two waves of impulse, having travelled paths of 

 unequal length, meet in opposite phases, and an 

 interference or inhibition results. A firm pressure 

 applied to the sole of the foot arouses no reflex con- 

 traction. No place is given in the theory to special 

 inhibitory cells. Any cell may exercise an inhibi- 

 tory action on the sensory or motor cells with which 

 it is connected. Whether its action on any other cell 

 shall augment or inhibit the activity of the latter, 

 depends on the phase in which the wave of impulse 

 travelling from it meets the wave of impulse that 

 has reached the same cell from another source. In 

 the case of inhibition by the will, the impulse sent 

 down from the brain is supposed to interfere with 

 that originating in the cord from the stimulation of 

 sensory nerves. Besides inhibition by interference, 

 apparent inhibition by the diversion of the stimulus 

 into otlier than its customary path may occur. 



Brunton attempts to explain many of the well- 

 known phenomena of inhibition on this hypothesis. 

 His explanation of the action of drugs — such as 

 atropia, morphia, strychnia — on the theory of inter- 

 ference is particularly weak and unsatisfactory. — 

 (Nature, nos. 696-699.) w. h. h. [1177 



Man, 

 Electrotonus of the motor nerves of man. — 

 Since the discovery by Pfliiger of the general laws of 

 electrotonic changes in a nerve during the passage of 

 a galvanic current, from investigations made upon the 

 dissected nerves of frogs, numerous attempts have 

 been made to verify his conclusions for the uninjured 

 nerve of man. The general outcome of this work 

 has not been satisfactory, as far as a confirmation of 

 Pfliiger's generalizations is concerned. Perhaps the 

 chief cause of the discrepancy amongst the results of 

 different observers has been the neglect to fully ap- 

 preciate the fact pointed out by Helmholtz, that 

 when the uninjured nerve, in its natural position in 

 the body, is exposed to an electrical current, there 

 exist in the region of each electrode, owing to rapid 

 current diffusion, areas of different electrical density, 

 which must, therefore, be considered as electrodes of 

 opposite signs. Waller and de Watteville have inves- 

 tigated the subject anew upon the motor nerves of 

 man, and obtained results which are in accord with 

 the laws established by Pfliiger. Their experiments 

 were^made in most cases upon the peroneal nerve, 

 and the contractions of the corresponding muscles 

 were registered by appropriate means upon a smoked 

 drum. They employed three methods of stimula- 

 tion, — induction currents, constant currents, and 

 mechanical stimuli. The unipolar method was used 

 in all cases, and the polarizing and stimulating cur- 

 rents were combined in one circuit. By this means 

 the points of stimulation and polarization were made 

 co-extensive, and the electrotonic changes in the polar 

 region obtained. lu mechanical excitation the same 

 result was reached by using the polarizing electrode 

 itself to give the stimulating blow. The authors have 



