1 6 POPULAR SCIENCE MONTHLY 



the long recognized arteries, veins and capillaries, there is another 

 class of blood vessels of great importance. These vessels are called 

 sinusoids and to a certain extent resemble capillaries, but their de- 

 velopment and their relation to the parts of the organs in which they 

 occur are entirely different from those of true capillaries. A sinusoid 

 is developed by the subdivision of a single large vessel. It consists 

 of endothelium, but that endothelium rests directly, or almost directly, 

 upon the cells of the organ in which this type of vessel occurs. Capil- 

 laries, on the other hand, are developed as small buds from preexisting 

 vessels and are always found in connective tissue. The most important 

 organ in which sinusoids occur is the liver, and the peculiar circulatory 

 arrangements in that organ, which have so long seemed singular and 

 puzzling, have become comprehensible, and have acquired greater 

 significance since the conception ' sinusoid ' was established. This 

 newly formed conception has unlocked the mystery of the portal cir- 

 culation, and has explained the supply of venous blood to the liver. A 

 morphological explanation of the portal vein had previously remained 

 impossible. 



To the fact that embryology explains many anomalies of the adult 

 structure, we have already referred. Let us leave out of consideration 

 the true monstrosities and confine our attention for the present to the 

 anomalies, which are due to arrest of development. These are com- 

 paratively frequent, and many of them are so definite we may fairly 

 call them typical. Such, for instance, is the preservation in the adult 

 of the foramen ovale between the auricles of the heart, or of the open 

 ductus arteriosus by which blood of the pulmonary and body circulation 

 may mingle. In case of the veins also an arrangement of vessels is 

 often found in the adult which is due to the persistence of a truly 

 embryonic condition. The urogenital system seems to be peculiarly 

 subject to arrests of development. When it starts the rudiments for 

 both sexes are complete and the two sexes become differentiated largely 

 by the obliteration in the individual of one sex of those structures 

 which are characteristic of the other, but not infrequently it happens 

 that this law of suppression is disturbed, and we then get very inter- 

 esting and significant anomalies with which the physician often has 

 practically to deal. Such cases do not produce a true hermaphroditism. ' 

 That is a condition which apparently may occur in the human species, 

 but is of the utmost rarity. Among other of the most frequent and 

 familiar illustrations of arrested development I will mention cleft 

 palate and hair lip. It is quite unnecessary to prolong this list, for 

 it is evident that the anomalies we are considering are of a definite 

 prescribed nature. They are all of practical importance to the physi- 

 cian, and unless he knows something of embryology he can not know 

 what these probable anomalies are. If he does know something of 



